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How Digital Cameras Work
by Karim Nice and Gerald Jay Gurevich

A Nikon digital camera
Megapixel digital cameras are coming down in price, while the quality and number of features of these cameras keeps going up. Some projections suggest that digital cameras will become as popular as film-based cameras by 2005. In this edition of HowStuffWorks, you will learn how a digital camera converts light into an image that can be stored on a computer or printed out on a desktop printer.

You may want to start with "How It Works" to learn the basics of how a digital camera works -- it's an amazing system! If you've never used a digital camera before, check out "What It Can Do." "Features" tells you about all of the features you find on digital cameras so you know what you are talking about if you are looking to buy one. Just click on the different areas to learn all about these amazing devices!

The following map leads you to all of the available information!

Understanding the Basics
Let's say you want to take a picture and e-mail it to a friend. The first step is to create a digital version of the image, so your computer can process it. (For information on sampling and digital representations of data, see this page explaining the digitization of sound waves -- digitizing light waves works in a similar way.) There are two commonly available methods for creating a digital image:

  • Take a photograph using a film emulsion, process it chemically, print it onto photographic paper and then use a digital scanner to sample the print. For an explanation of photographic film, see the article How Photographic Film Works.
  • Use a device that will sample the original light that bounces off your subject to create a digital image. This device is called a digital camera. Sometimes, it is referred to as a filmless camera.
A digital camera is the easier and faster path to take. Let's look at the sensor in a digital camera that captures the image...

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A Filmless Camera
The key difference between a digital camera and a film-based camera is that the digital camera has no film. Instead, it has a sensor that converts light into electrical charges. All the fun and interesting features of digital cameras come as a direct result of this shift from recording an image on film to recording the image in digital form.

The image sensor employed by most digital cameras is a charge coupled device (CCD). Some low-end cameras use complementary metal oxide semiconductor (CMOS) technology. While CMOS sensors will almost certainly improve and become more popular in the future, they probably won't replace CCD sensors in higher-end digital cameras. Throughout the rest of this article, we will mostly focus on CCD. For the purpose of understanding how a digital camera works, you can think of them as nearly identical devices. Most of what you learn will also apply to CMOS cameras.

The CCD is a collection of tiny light-sensitive diodes, which convert photons (light) into electrons (electrical charge). These diodes are called photosites. In a nutshell, each photosite is sensitive to light -- the brighter the light that hits a single photosite, the greater the electrical charge that will accumulate at that site.

Figure 1. Photons hitting a photosite and creating electrons

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The Difference Between CCD and CMOS
As prices continue to fall, more and more people are using digital cameras. The cameras are not as common as film cameras yet, but things are certainly moving in that direction. One of the drivers behind the falling prices has been the introduction of CMOS image sensors. CMOS sensors are much less expensive to manufacture than CCD sensors.

A CMOS image sensor
Both CCD and CMOS image sensors start at the same point -- they have to convert light into electrons at the photosites. If you've read the article How Solar Cells Work, you already understand one of the pieces of technology used to perform the conversion. A simplified way to think about the sensor used in a digital camera (or camcorder) is to think of it as having a 2-D array of thousands or millions of tiny solar cells, each of which transforms the light from one small portion of the image into electrons. Both CCD and CMOS devices perform this task using a variety of technologies.

The next step is to read the value (accumulated charge) of each cell in the image. In a CCD device, the charge is actually transported across the chip and read at one corner of the array. An analog-to-digital converter turns each pixel's value into a digital value. In most CMOS devices, there are several transistors at each pixel which amplify and move the charge using more traditional wires. The CMOS approach is more flexible because each pixel can be read individually.

CCDs use a special manufacturing process to create the ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CMOS chips, on the other hand, use completely normal manufacturing processes to create the chip -- the same processes used to make most microprocessors. Because of the manufacturing differences, there are several noticeable differences between CCD and CMOS sensors.

  • CCD sensors, as mentioned above, create high-quality, low-noise images. CMOS sensors, traditionally, are more susceptible to noise.
  • Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip is lower. Many of the photons hitting the chip hit the transistors instead of the photodiode.
  • CMOS sensors traditionally consume little power. Implementing a sensor in CMOS yields a low-power sensor.
  • CCDs, on the other hand, use a special process that consumes lots of power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.
  • CMOS chips can be fabricated on just about any standard silicon production line, so they tend to be extremely inexpensive compared to CCD sensors.
  • CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality pixels, and more of them.

Based on these differences, you can see that CCDs tend to be used in cameras that focus on high-quality images with lots of pixels and excellent light sensitivity. CMOS sensors usually have have lower quality, lower resolution and lower sensitivity. However, CMOS cameras are much less expensive and have great battery life. Over time, CMOS sensors will improve to the point where they reach near parity with CCD devices in most applications, but they are not there yet.

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How the Camera Captures Color
Unfortunately, each photosite is colorblind. It only keeps track of the total intensity of the light that strikes its surface. In order to get a full color image, most sensors use filtering to look at the light in its three primary colors. Once all three colors have been recorded, they can be added together to create the full spectrum of colors that you've grown accustomed to seeing on computer monitors and color printers.

Figure 2. How the three colors mix to form many colors

There are several ways of recording the three colors in a digital camera. The highest quality cameras use three separate sensors, each with a different filter over it. Light is directed to the different sensors by placing a beam splitter in the camera. Think of the light entering the camera as water flowing through a pipe. A beam splitter would be like dividing an identical amount of water into three different pipes. Each sensor gets an identical look at the image, but because of the filters, they only respond to one of the primary colors.

Figure 3. How the original (left) image is split in a beam splitter

The advantage of this method is that the camera records each of the three colors at each pixel location. Unfortunately, cameras that use this method are both bulky and expensive.

A second method is to rotate a series of red, blue and green filters in front of a single sensor. The sensor records three separate images in rapid succession. This method also provides information on all three colors at each pixel location. But since the three images aren't taken at precisely the same moment, both the camera and the target of the photo must remain stationary for all three readings. This isn't practical for candid photography or handheld cameras.

Figure 4. A spinning disk filter

A more economical and practical way to record the three primary colors from a single image is to permanently place a filter over each individual photosite. By breaking up the sensor into a variety of red, blue and green pixels, it is possible to get enough information in the general vicinity of each sensor to make very accurate guesses about the true color at that location. This process of looking at the other pixels in the neighborhood of a sensor and making an educated guess is called interpolation. (You'll learn more about pixels in the next section, but for now, think of one photosite as a single pixel.)

The most common pattern of filters is the Bayer filter pattern. This pattern alternates a row of red and green filters with a row of blue and green filters. You may be surprised to find that the pixels are not evenly divided. In fact, there are as many green pixels as there are blue and red combined. This is because the human eye is not equally sensitive to all three colors. It's necessary to include more information from the green pixels in order to create an image that the eye will perceive as a "true color."

Figure 5

The advantages of this method are that only one sensor is required and all the color information (red, green and blue) is recorded at the same moment. That means the camera can be smaller, cheaper and useful in a wider variety of situations. In other words, it makes it possible to create an affordable handheld digital camera. The raw output from a sensor with a bayer filter is a mosaic of red, green and blue pixels of different intensity.

If you took a close look at Figure 5, you might be wondering how a digital camera can achieve its full advertised resolution if it takes 4 separate pixels to determine the color of a single pixel. Digital cameras use specialized demosaicing algorithms to convert the mosaic of separate colors into an equally sized mosaic of true colors. The key is that each colored pixel can be used more than once. The true color of a single pixel can be determined by averaging the values from the closest surrounding pixels.

Figure 6. A demosaicing algorithm at work

There are other ways of handling color in a digital camera. Some single-sensor cameras use alternatives to the bayer filter pattern. There is even a method that uses two sensors. Some of the advanced cameras don't add up the different values of red, green and blue, but instead subtract values using the typesetting colors cyan, yellow, green and magenta. However, most consumer cameras on the market today use a single sensor with alternating rows of green-red and green-blue filters.

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Some Technical Details and a Word About Resolution
In the previous section, we glossed over one of the important technical details so that we could simplify the explanation of color. You've learned that light is converted to electrical charge, but the electrical charges that build up in the CCD are not digital signals that are ready to be used by your computer. In order to digitize the information, the signal must be passed through an analog-to-digital converter (ADC). Interpolation is handled by a microprocessor after the data has been digitized.

Figure 7. The whole process

Think of each photosite as a bucket or a well. Now think of the photons of light as raindrops. As the raindrops fall into the bucket, water accumulates (in reality, electrical charge accumulates). Some buckets have more water and some buckets have less water, representing brighter and darker sections of the image. Keeping to the analogy, the ADC measures the depth of the water, which is considered analog information. Then it converts that information to binary form.

Is the number of photosites the same as the number of pixels?
If you read digital camera claims carefully, you'll notice that the number of pixels and the maximum resolution numbers don't quite compute. For example, a camera claims to be a 2.1-megapixel camera and it is capable of producing images with a resolution of 1600 X 1200. Let's do the math, a 1600 x 1200 image contains 1,920,000 pixels. But "2.1 megapixel" means there ought to be at least 2,100,000 pixels. This isn't an error from rounding off, and it isn't binary mathematical trickery. There is a real discrepancy between these two numbers. If a camera says it has 2.1 megapixels, then there really are approximately 2,100,000 photosites on the CCD.

What happens is that some of the photosites are not being used for imaging. Remember that the CCD is an analog device. It's necessary to provide some circuitry to the photosites so that the ADC can measure the amount of charge. This circuitry is dyed black so that it doesn't absorb any light and distort the image.

How big are the sensors?
The current generation of digital sensors are smaller than film. Typical film emulsions that are exposed in a film-based camera measure 24 mm x 36 mm. If you've look at the specifications of a typical 1.3-megapixel camera, you'll find that it has a CCD sensor that measures 4.4 mm x 6.6 mm. As you'll see in a later section, a smaller sensor means smaller lenses.

You should keep abreast of the current literature, though, because things change fast in this field. At the time of this writing, there is a CCD sensor that contains over 6 million photosites and is approximately the size of standard 35 mm film.

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Output, Storage and Compression
Most digital cameras on the market today have an LCD screen, which means that you can view your picture right away. This is one of the great advantages of a digital camera: You get immediate feedback on what you capture. Once the image leaves the CCD sensor (by way of the ADC and a microprocessor), it is ready to be viewed on the LCD.

Of course, that's not the end of the story. Viewing the image on your camera would lose its charm if that's all you could do. You want to be able to load the picture into your computer or send it directly to a printer. There are several ways to store images in a camera and then transfer them to a computer.

Early generations of digital cameras had fixed storage inside the camera. To get the pictures out, they needed to be hooked up directly to a computer by cables so that the images could be transferred. Although most of today's cameras are capable of connecting to a serial, parallel, SCSI, and/or USB ports, they usually provide you with some sort of removable storage device. The main rival technologies are CompactFlash, SmartMedia and Memory Sticks, but there are others. These three are all small, removable, solid-state Flash memory devices that have no moving parts. They are fast, inexpensive ways of storing your photos so that you can transfer them to a computer or printer later on.

In order to transfer the files from a Flash memory device to your computer without using cables, you will need to have a drive or reader for your computer. These devices behave much like floppy drives and are inexpensive to buy.

Think of all these storage devices as reusable digital film. When you fill one up, either transfer the data, or put another one into the camera. The different types of Flash memory devices are not interchangeable. Each camera manufacturer has decided on one device or another. Each of the Flash memory devices also needs some sort of caddy or card reader in order to transfer the data.

One notable example of a camera that does not use Flash memory is the Sony Mavica series of cameras. They use standard floppy disks, which almost all computers can read.

It takes a lot of memory to store a picture with over 1.2 million pixels. Almost all digital cameras use some sort of data compression to make the files smaller. There are two features of digital images that make compression possible. One is repetition. The other is irrelevancy.

You can imagine that throughout a given photo, certain patterns develop in the colors. For example, if a blue sky takes up 30 percent of the photograph, you can be certain that some shades of blue are going to be repeated over and over again. When compression routines take advantage of patterns that repeat, there is no loss of information and the image can be reconstructed exactly as it was recorded in the camera. Unfortunately, this doesn't reduce files any more than 50 percent, and sometimes it doesn't even come close to that level.

Irrelevancy is a trickier issue. A digital camera records more information than is easily detected by the human eye. Some compression routines take advantage of this fact to throw away some of the more meaningless data. If you need smaller files, you need to be willing to throw away more data. Most cameras offer several different levels of compression, although they may not call it that. More likely they will offer you different levels of resolution. This is the same thing. Lower resolution means more compression. And more compression means lower resolution.

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Controlling the Amount of Light That Reaches the Sensor
It is important to control the amount of light that reaches the sensor. Thinking back to the water bucket analogy, if too much light hits the sensor the bucket will fill up and won't be able to hold any more. If this happens, information about the intensity of the light is being lost. Even though one photosite may be exposed to a higher intensity light than another, if both buckets are full, the camera will not register a difference between them.

The word camera comes from the term camera obscura. Camera means room (or chamber) and obscura means dark. In other words, a camera is a dark room. This dark room keeps out all unwanted light. At the click of a button, it allows a controlled amount of light to enter through an opening and focuses the light onto a sensor (either film or digital). In this section you will learn how the aperture and shutter work together to control the amount of light that enters the camera.

The aperture is the size of the opening in the camera. It's located behind the lens. On a bright sunny day, the light reflected off your image may be very intense, and it doesn't take very much of it to create a good picture. In this situation, you want a small aperture. But on a cloudy day, or in twilight, the light is not so intense and the camera will need more light to create an image. In order to allow more light, the aperture must be enlarged.

Your eye works the same way. When you are in the dark, the iris of your eye dilates your pupil (that is, it makes it very large). When you go out into bright sunlight, your iris contracts and it makes your pupil very small. If you can find a willing partner and a small flashlight, this is easy to demonstrate (if you do this, please use a small flashlight, like the ones they use in a doctor's office). Look at your partner's eyes, then shine the flashlight in and watch the pupils contract. Move the flashlight away, and the pupils will dilate.

Shutter Speed
Traditionally, the shutter speed is the amount of time that light is allowed to pass through the aperture. Think of a mechanical shutter as a window shade. It is placed across the back of the aperture to block out the light. Then, for a fixed amount of time, it opens and closes. The amount of time it is open is the shutter speed. One way of getting more light into the camera is to decrease the shutter speed. In other words, leave the shutter open for a longer period of time.

Film-based cameras must have a mechanical shutter. Once you expose film to light, it can't be wiped clean to start again. Therefore, it must be protected from unwanted light. But the sensor in a digital camera can be reset electronically and used over and over again. This is called a digital shutter. Some digital cameras employ a combination of electrical and mechanical shutters.

Exposing the Sensor
These two aspects of a camera, aperture and shutter speed, work together to capture the proper amount of light needed to make a good image. In photographic terms, they set the exposure of the sensor. Most digital cameras automatically set aperture and shutter speed for optimal exposure, which gives them the appeal of a point-and-shoot camera.

Some digital cameras also offer the ability to adjust the aperture settings by using menu options on the LCD panel. More advanced hobbyists and professionals like to have control over the aperture and shutter speed selections because it gives them more creative control over the final image. As you climb into the upper levels of consumer cameras and the realm of professional cameras, you will be rewarded with controls that have the look, feel and function common to film-based cameras.

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Lens and Focal Length
A camera lens collects the available light and focuses it on the sensor. Most digital cameras use auto-focus techniques, which you can learn more about in the article How Autofocus Cameras Work

The important difference between the lens of a digital camera and the lens of a 35 mm camera is the focal length. The focal length is the distance between the lens and the surface of the sensor. You learned in the section on technical details that the surface of a film sensor is much larger than the surface of a CCD sensor. In fact, a typical 1.3 megapixel digital sensor is approximately one-sixth of the linear dimensions of film. In order to project the image onto a smaller sensor, it is necessary to shorten the focal length by the same proportion.

Focal length is also the critical information in determining how much magnification you get when you look through your camera. In 35 mm cameras, a 50 mm lens gives a natural view of the subject. As you increase the focal length, you get greater magnification and objects appear to get closer. As you decrease the focal length, things appear to get further away, but you can capture a wider field of view in the camera.

Since, many photographers that use film-based cameras are familiar with the focal lengths that project an image onto 35 mm film, digital cameras advertise their focal lengths with "35 mm equivalents." This is extremely helpful information to have. In the chart below, you can compare the actual focal lengths of a typical 1.3-megapixel camera and its equivalent in a 35 mm camera.

Focal Length 35 mm Equivalent View Typical Uses
5.4 mm
35 mm
Things look smaller and farther away. Wide-angle shots, landscapes, large buildings, groups of people
7.7 mm
50 mm
Things look about the same distance as what your eye sees. "Normal" shots of people and objects
16.2 mm
105 mm
Things are magnified and appear closer. Telephoto shots, close-ups

Optical Zoom vs. Digital Zoom
In general terms, a zoom lens is any lens that has an adjustable focal length. Zoom doesn't always mean a close up. As you can see in the chart above, the "normal" view of the world for this particular camera is 7.7 mm. You can zoom out for a wide-angle view of the world, or you can zoom in for a closer view of the world. Digital cameras may have an optical zoom, a digital zoom, or both.

An optical zoom actually changes the focal length of your lens. As a result, the image is magnified by the lens (sometimes called the optics, hence optical zoom). With greater magnification, the light is spread across the entire CCD sensor and all of the pixels can be used. You can think of an optical zoom as a true zoom that will improve the quality of your pictures.

A digital zoom is a computer trick that magnifies a portion of the information that hits the sensor. Let's say that you are shooting a picture with a 2X digital zoom. The camera will use half of the pixels at the center of the CCD sensor and ignore all the other pixels. Then it will use interpolation techniques to add detail to the photo. Although it may look like you are shooting a picture with twice the magnification, you can get the same results by shooting the photo without a zoom and blowing up the picture using your computer software.

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How It Works: Summary
If you've made it this far, you've learned a lot about digital cameras. Let's put it all together as we take a picture to send to friends.

  • Aim the camera at the subject and adjust the optical zoom to get closer or further away.
  • Press lightly on the shutter release.
  • The camera automatically focuses on the subject and takes a reading of the available light.
  • The camera sets the aperture and shutter speed for optimal exposure.
  • Press the shutter release all the way.
  • The CCD is reset and then exposed to the light, building up an electrical charge, until the shutter closes.
  • The ADC measures the charge and creates a digital signal that represents the values of the charge at each pixel.
  • A processor interpolates the data from the different pixels to create natural color. On many cameras, it is possible to see the output on the LCD at this stage.
  • A processor may perform a preset level of compression on the data.
  • The information is stored on some form of memory device (probably a Flash memory card).
  • The picture is transferred to a computer or a printer.
  • The picture can be attached to an e-mail message or posted to a Web page.

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What They Can Do
Digital cameras come in forms small enough and versatile enough to perform almost all of the tasks that conventional cameras can perform, and they can do quite a few things that conventional cameras cannot.

The digital camera is ideal for creating pictures that you can e-mail to friends or post on the Internet.

The digital camera can be used much like a conventional film camera. Most digital cameras have more in common with automatic point-and-shoot cameras than they do with professional SLR cameras. They generally have auto-focus, and they can adjust for brightness, shutter speed and aperture automatically.

The "film" of a digital camera is a removable media-storage device (floppy disk, Flash memory card, etc). As with a film camera, you simply replace the storage device when it's full and continue to take pictures. The difference is you don't need to develop digital pictures. You can download them directly to your computer and they are ready to use.

E-mailing Pictures to Friends
If you take pictures in JPEG format at 640 x 480 resolution, you can download them to your computer and e-mail them to friends without having to do anything to the picture. This eliminates most of the steps you would need to take to do this with a film camera: There's no need to get film developed or scan the developed picture. Just take the picture, transfer it to the computer and e-mail it.

Posting Pictures on the Web
The files that you create with your digital camera can also be posted to the Web. If you have a personal home page, you can display your pictures there.

Another good use of the digital camera is taking pictures of things you would like to sell. A digital camera can save you a lot of time and money if you often sell things in online auctions because you can take a picture of an object and post it to the auction site very quickly and without having to pay for film or developing.

If you're selling a house or car, you could also post pictures on the Web to help potential buyers see what they are getting.

Creating Fun Photos
With the image-editing software that often comes with your camera you can do lots of neat things. You can:

  • Crop the picture to capture just the part you want
  • Add text to the picture
  • Make the picture brighter or darker
  • Change the contrast and sharpness
  • Apply filters to the picture to make it look blurry, painted, embossed, etc.
  • Resize pictures
  • Rotate pictures
  • Cut stuff out of one picture and put it into another

Creating Large Panoramas
Some digital cameras and software programs can "stitch" together many pictures to create one large panoramic picture. This feature could be used to create a 360-degree view from the top of a mountain, or the inside of a room in your house.

Some software can even create a 3-D picture that you can rotate and zoom in on or out of.

If you are looking for a digital camera, there are a bunch of different features you need to know about so that you can pick the perfect camera for you. By learning about all of the different features you will be able to have intelligent conversations with sales people and friends, and you will also be able to decode all of the jargon you see in catalogs and store displays!

Here are the most important features you need to think about when choosing a digital camera:

Click on any feature above to learn about it, or choose from the map below:

The amount of detail that the camera can capture is called the resolution, and it is measured in pixels. In general, the more pixels your camera has, the more detail it can capture. The more detail you have, the more you can blow up a picture before it becomes "grainy," and starts to look out-of-focus.

Some typical resolutions that you find in digital cameras today include:

  • 256x256 pixels - You find this resolution on cheap cameras, like those you would buy for a child. This resolution is so low that the picture quality is almost always unacceptable. This is 65,000 total pixels.
  • 640x480 pixels - This is the low end on most "real" cameras. This resolution is great if you plan to e-mail most of your pictures to friends or post them on a Web site. This is 307,000 total pixels.
  • 1216x912 pixels - If you are planning to print your images, and want them to look nice, this is a good resolution. This is a "megapixel" image size -- 1,109,000 total pixels.
  • 1600x1200 pixels - This is "high resolution." Images taken with this resolution can be printed in larger sizes, like 8 inches x 10 inches, with good results. This is almost 2 million total pixels. You can find cameras today with up to 3 million pixels.

You may or may not need lots of resolution, depending on what you want to do with your pictures. If you are planning to do nothing more than display images on a Web page or send them in e-mail, then using 640x480 resolution has several advantages:

  • Your camera's memory will hold more images at this low resolution than at higher resolutions.
  • It will take less time to move the images from the camera to your computer.
  • The images will take up less space on your computer.
On the other hand, if your goal is to print large images, you definitely want to take high resolution shots, and need a camera with lots of pixels.

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Sensor Technology
There are two different sensors used on cameras today:

  • CCD sensors - CCD sensors have been around longer. In general, CCDs have more pixels and work better in low light. However, they tend to be more expensive and they use a lot of power.
  • CMOS sensors - CMOS sensors are lower resolution, use less power and do not work well in low light. If you are looking for an inexpensive, low-resolution camera, and plan to use it outdoors, a CMOS sensor would be a good choice.
High-resolution cameras need a CCD sensor. See this section for details.

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Your camera needs to store its images somewhere, and it uses memory to do that. There are a number of storage systems currently used in digital cameras:

  • Built-in memory - Some extremely inexpensive cameras have built-in Flash memory. There is no way for you to change or upgrade the memory. The built-in memory might be able to hold six to 12 images; then you must either download the images to your computer or erase images before you can take more.
  • SmartMedia cards - SmartMedia cards are small Flash memory modules. The camera will come with one, and then you can purchase additional cards if you want more memory. Cards come in 4, 8, 16, 32, 64 and 128 megabyte (MB) sizes.
  • CompactFlash - CompactFlash cards are another form of Flash memory, similar to but slightly larger than SmartMedia cards. Capacities range up to 128 MB.
  • Memory Stick - Memory Stick is a proprietary form of Flash memory used by Sony.
  • Floppy disk - Some cameras store images directly onto floppy disks. This makes image transfers easy because you can just pop a diskette out of the camera and into any computer. The problem is that a floppy disk only holds 1.4 MB of information, which is not much when you're taking high-resolution images. You can fit about 30 640x480-pixel images on a floppy disk.
  • Hard disk - Some higher-end cameras use small built-in hard disks, or PCMCIA hard-disk cards, for image storage. Sizes can range up to a gigabyte (GB), depending on the form factor.
  • Writeable CD and DVD - Some of the newest cameras are using writeable CD and DVD drives to store images. A writeable CD can hold 650 MB; a writeable DVD can hold 5.6 gigabytes (GB).
To choose between all of these different options, here are some things to consider:
  • If you are the sort of person who wants to take a dozen 640x480 snapshots at a birthday party and then upload them to your computer to e-mail them, any of these systems will do. You don't need much memory, so a floppy disk is fine, or a small amount (2 or 4 MB) of Flash memory works great.
  • If you are a person who plans to go on a trip, take hundreds of pictures and then come home to look at them all, you need lots of storage space. You can do that by purchasing extra Flash memory cards or by using a camera with a hard disk.
  • If you want to take high-resolution images (for example, 2-megapixel images), you need lots of storage. Purchase extra Flash memory cards or use a camera with a hard disk.

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Compression Formats
Most of today's cameras store their images in JPEG format; and you might be able to select between "fine detail mode" and "normal mode." Higher-end cameras may also support the TIFF format. While JPEG compresses the image, TIFF does not, so TIFF images take lots of space. The advantage of TIFF storage is that no data is lost to the compression process.

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Many digital cameras come with an LCD. The LCD (liquid crystal display) is often used as a viewfinder. It also allows you to review images in memory. You can then delete images that you don't like, and take more pictures in their place.

Some newer cameras have advanced features for the LCD. For example, you can zoom in on parts of the image and see them in greater detail.

Check to see how the LCD looks both in darkness (it needs a backlight to work in the dark) and bright sunlight. For details on LCDs, see How LCDs Work.

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View Finder
Some cameras have no LCD panel, and instead use a simple optical viewfinder. Other cameras have both an LCD panel and an optical viewfinder, in which case you can turn off the LCD to save the battery. Some cameras have only an LCD panel, which also acts as the viewfinder.

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You will find four different types of lenses on digital cameras:

  • Fixed-focus, fixed-zoom lenses - These are the kinds of lenses you find on disposable and inexpensive film cameras -- inexpensive and great for snapshots, but fairly limited.

  • Optical-zoom lenses with automatic focus - similar to the lens on a video camcorder, you have "wide" and "telephoto" options and automatic focus. The camera may or may not let you switch to manual focus.

  • Digital-zoom lenses - With digital zoom, the camera takes pixels from the center of the image sensor and "interpolates" them to make a full-size image. Depending on the resolution of the image and the sensor, this approach may create a grainy or fuzzy image. It turns out that you can do the same thing a digital zoom is doing on your own -- simply snap a picture and then cut out the center of the image in your image processing software.

  • Replaceable lens systems - If you are familiar with high-end 35 mm cameras, then you are familiar with the concept of replaceable lenses. High-end digital cameras can use this same system, and in fact can use lenses from 35 mm cameras in some cases.

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The interface allows you to connect the camera to your computer and transfer the images. Once the images are on your computer, you can edit them, e-mail them, load them onto a Web site or print them out. There are at least four different ways to move images from the camera to the computer:

  • USB connection - A USB connection is a fast, easy way to download images. It works only on computers that have a USB port.
  • Serial connection - This is a slow way to download images, but works on almost any computer. You often have to load special software onto the computer to download the images.
  • Floppy disk - If your camera uses a floppy disk (or writeable CD/DVD), then you can simply pop the floppy disk into any computer.
  • Flash memory slots - It is possible to add a box to your computer that will let you remove a SmartMedia card or CompactFlash card from the camera and attach it directly to the computer.

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Exposure Control
Most cameras have completely automatic exposure. If you want exposure control, look for a manual exposure feature. Some cameras also offer special exposure settings for specific situations like sports, indoor arenas, etc.

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Image Stabilization
Some cameras offer a "steady cam" image stabilization system like video cameras have (see How Steadicams Work). A system like this can help you take clearer images.

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Most cameras have fixed focus, so you can not adjust the focus at all, or automatic focus. If you would like more control, choose a camera with a manual focus feature.

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If you plan to take close-up images, look for a camera that has a macro focusing capability. This feature lets you move the camera's lens very close to the subject. Here is an example of a macro photograph -- in this case this is a part of a small electric motor, and the white disk is about the size of a U.S. quarter coin:

If your camera is not equipped with a macro setting, there is no way for it to take an image like this.

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Some cameras now offer an "MPEG movie" feature, allowing you to take short movies with your camera. If this is a feature that you want, make sure the camera offers it because this is not a standard feature. Look for the ability to change the resolution of the movie, and find out the maximum movie length you can record.

To see an example of a typical movie taken by a digital camera, click on the image below. This movie comes from this Question of the Day and demonstrates a trick birthday candle:

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Digital cameras, especially those that use a CCD sensor and an LCD display, tend to use lots of power -- which means they eat batteries. Rechargeable batteries help to lower the cost of using the digital camera, but rechargeable batteries are sometimes expensive. Here are some things to consider:

  • Does the camera use standard-size rechargeable batteries (e.g., AA), or does it use special rechargeable batteries made by the manufacturer? If it uses the special ones, check to see what the price of another battery pack is.
  • If the camera takes AA batteries, can you use normal alkaline batteries in a pinch?
  • Are the rechargeable batteries removable, or are they permanently mounted in the camera? If they are not removable, it means that once the batteries go dead you can't use the camera again until you can get to a recharger and power supply. This can be a major pain in the neck if you want to take a lot of pictures at once.

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Most cameras come with a built-in flash. Normal modes are:

  • Automatic
  • Forced flash
  • Fill flash
  • No flash
You definitely want to be able to turn off the flash -- some completely automatic cameras do not allow this.

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Most cameras come with some sort of software. You use the software to download images from the camera and manipulate them on the computer. Unless you already have software that you are comfortable with, you want to make sure that your camera comes with something.

When You Shop
We've created a feature comparison chart for you to use as you research various digital cameras. Take it to the store with you and fill in the blanks for each model you are interested in. You may also want to keep an additional copy near your desk as you research digital cameras on the Internet.

The feature comparison chart is available to you as a PDF. You will need the free Adobe Acrobat Reader to view it.

To give you an example of typical cameras, here are some popular ones:

Kodak Easyshare DX4900 Zoom

Sony Mavica MVC-FD100

Kodak DC3800

Canon PowerShot S100 Digital ELPH Camera Kit

Canon PowerShot S20 Digital Camera Kit

Nikon Coolpix 800

Olympus D460 Zoom

Sony Mavica MVC-FD95

Nikon Coolpix 990

Olympus Camedia D-360L

Look Out!
When purchasing a digital camera, there are several things you should keep in mind to avoid buying a camera that won't meet your needs. Here are some of the most common things to think about:

Make sure the camera has the right resolution for your needs.
If you are going to take snapshots and e-mail them to friends, then you don't need anything more than 640x480 pixel resolution. Buying the resolution that you need lets you save money (and hard disk space). On the other hand, if you want to print enlarged versions of your photos, a 2- or 3-megapixel camera is what you have to have, and you need to be willing to pay for the pixels.

Make sure the camera has enough memory.
There is nothing more frustrating that "running out of film" when there is a great picture sitting in your viewfinder! The "film" for a digital camera is Flash memory, floppy disks, small hard disks, etc. Most cameras let you download pictures from the camera so that you can take more, but if you go on a week-long vacation you will be away from your computer and won't be able to download. So make sure you pick up enough extra memory when you buy your camera so you won't run out when you need it.

Make sure the lens will handle the pictures you plan to take.
If you don't have the right lens, it can be hard to take the best pictures. For example, if you like to take close-up pictures but your camera does not have a macro setting, you won't be able to take close-ups. If your camera has a 2X zoom but you really need a 10X zoom for a lot of your pictures, that can be a big problem too. Be sure to try out the lens system on a camera before you purchase it. Digital cameras come with a huge variety of lenses, so be sure to shop around.

Do not confuse digital zoom with optical zoom.
Many cameras advertise things like "100X zoom," but that is often misleading because only part of it is in the lens. The only part of a zoom lens that really matters is the "optical" part -- the part made out of glass lenses. Any form of "Digital Zoom" is something you can do yourself outside of the camera. If you use your camera's software to crop out a small inner portion of a picture and blow it up, you are doing the same thing a digital zoom is doing. Almost always the digital zoom simply makes the image fuzzy.

Do not confuse actual resolution with interpolated resolution.
Many cameras will advertise that they have, for example, 1000 x 600 pixel resolution and 1200 x 800 interpolated resolution. Like digital zoom, interpolated resolution is an illusion. You could do the same thing yourself with the camera's software, and it would do nothing but make the image larger and slightly fuzzy.

See how long the batteries will last.
Many digital cameras eat batteries because they have to power an image sensor, an LCD panel and a microprocessor all at the same time, and sometimes there's a flash as well! See how long the batteries will really last in your camera. See if the camera will accept normal alkaline batteries in a pinch. If you plan on using your camera for long periods of time, think about purchasing an extra battery for it and be sure to check prices ahead of time. Some manufacturers charge an arm and a leg for their batteries, and you may want to consider a different manufacturer.

Where to Buy

Places to buy digital cameras include:

In Association with



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What is the image capacity of each type of storage?
Right now there are two main types of storage media in use today. Some cameras use 1.44-MB floppy disks and some use various forms of Flash memory that have capacities ranging from 8 to 128 MB. There are several other formats, but for now we'll discuss these two.

The main difference between storage media is their capacity, the capacity of a floppy disk is fixed, and the capacity of Flash memory devices is increasing all the time. This is fortunate because picture size is also increasing constantly, as higher resolution cameras become available.

The two main file formats used by digital cameras are TIFF and JPEG. TIFF is an uncompressed format and JPEG is a compressed format. Most cameras use the JPEG file format for storing pictures, and they sometimes offer quality settings such as high or medium quality. The following chart will give you an idea of the file sizes you might expect when you take different size pictures.

Image Size
(high quality)
(medium quality)
1.0 MB
300 KB
90 KB
1.5 MB
500 KB
130 KB
2.5 MB
800 KB
200 KB
6.0 MB
1.7 MB
420 KB

One thing that becomes apparent is that a 1.44-MB disk cannot hold very many pictures. In fact, at some image sizes you can't even fit one picture on the disk. But the floppy disk does have its uses. For Internet publishing and e-mailing pictures to friends you almost never need a picture bigger than 640x480, and you will almost always save it in JPEG form. In this case you might be able to fit 16 or so pictures on each disk.

If you are trying to store the biggest highest quality images you can, then you will want the highest capacity medium. A 128-MB Flash memory card could store more than 1,400 small compressed images. But it could also store 21 of the uncompressed 1600x1200 images. You would probably never use the whole 128 MB if you were just taking small pictures, but if you were taking the big pictures this would be the only way to go. The large capacity might also come in handy if you were going on a long trip and wanted to be able to take lots of pictures.

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What are the advantages and disadvantages of the different storage types?
Aside from the number of images a medium can store, the main feature of a medium is how easy it is to get the files on and off.

Almost every computer has a 3.5-inch disk drive, so it is very easy to put the disk into your computer and transfer the files. But these disks are slow compared to Flash memory devices.

If you want to be able to pop the Flash memory into your computer, you will need a special drive that accepts the type of media your camera uses. These are generally inexpensive, but most computers do not come with them. If you don't have one of these drives, then you have to use the cable to transfer files between the camera and the computer.

For large files, Flash memory media is superior because of its fast transfer speed. This will reduce both the time it takes for the camera to store the image after you take it and the time it takes to transfer the image to your computer.

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What determines picture quality?
This is a pretty complicated question. The three main factors that affect image quality are:

  • The type of technology used to capture the initial image
  • The type of compression used to store the image
  • The resolution of the sensor

Capturing the Image
There are three basic ways that cameras capture the initial image. If you read How Digital Cameras Work, then you learned about the different types of filters and the difference between CCD and CMOS image sensors.

  • The best cameras have a light splitter, which splits the incoming light into its three component colors red, green and blue, and directs each onto its own CCD sensor. With this kind of camera, the exact color of each pixel is captured.

  • Most consumer cameras have one CCD chip and use a filter that allows each pixel to see only one of the three colors. This means that the exact color of each pixel is not captured; it must be calculated using the information from surrounding pixels. There are many different schemes for performing these calculations, so image quality can vary.

  • Low-end cameras use a CMOS image sensor, which currently can not match the resolution or color quality of the CCD chips.

Storing and Compressing the Image
The highest image quality will be obtained if you store the image in an uncompressed format. This means that the color of each pixel is recorded exactly as the camera saw it. However, a high quality JPEG compressed image may be almost indistinguishable from an uncompressed image.

Cameras with small storage mediums like 3.5-inch diskette drives will tend to use fairly high compression in order to fit a reasonable number of images on a disk. Some may not even have the option to store uncompressed images.

Sensor Resolution
If you just plan to publish pictures to the Internet or e-mail them to friends, a camera that produces 640x480-pixel images stored in JPEG format will be adequate. But if you plan on printing the images, then you need the highest resolution possible.

The most common setting for computer monitors is a width of 800 pixels and a height of 600 pixels, so a 640x480 image will fill up most of the screen. But printers can print at a much higher resolution than monitors can display.

In general the highest resolution will produce the best quality images. If you plan to print your images, then resolution is even more important as we'll see in the next section.

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What picture resolution will give me the best quality prints on my inkjet printer?
There are many different technologies used in inkjet printers. In general, printer manufacturers will advertise the printer resolution in dots per inch (dpi). However, all dots are not created equal. One printer may place more drops of ink (black, cyan, magenta or yellow) per dot than another.

For instance, printers made by Hewlett Packard that use PhotoREt III technology can layer a combination of up to 29 drops of ink per dot, yielding about 3,500 possible colors per dot. This may sound like a lot, but most cameras can capture 16.8 million colors per pixel. So these printers cannot replicate the exact color of a pixel with a single dot. Instead it must create a grouping of dots that when viewed from a distance blend together to form the color of a single pixel.

The rule of thumb is that you can divide your printer's color resolution by about four to get the maximum picture quality out of your printer. So for a 1200 dpi printer, a resolution of 300 pixels per inch would provide just about the best quality that printer is capable of. This means that with a 1200x900 pixel image, you could print a 4-inch by 3-inch print. In practice, though, lower resolutions than this usually provide adequate quality. To make a reasonable print that comes close to the quality of a traditionally developed photograph you need about 150 to 200 pixels per inch of print size.

On this page, Kodak recommends the following as minimum resolutions for these different print sizes.

Print Size
Image Resolution
640x480 pixels
4x5 inches
768x512 pixels
5x7 inches
1152x768 pixels
8x10 inches
1536x1024 pixels

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Why is the number of megapixels bigger than the number of pixels?
If you read digital camera claims carefully, you'll notice that the number of pixels and the maximum resolution numbers don't quite compute. For example, a camera claims to be a 2.1-megapixel camera and it is capable of producing images with a resolution of 1600x1200. Let's do the math. A 1600x1200 image contains 1,920,000 pixels. 2.1 megapixels means there ought to be at least 2,100,000 pixels. This isn't an error from rounding off. And it isn't binary mathematical trickery. There is a real discrepancy between these two numbers. If a camera says it has 2.1 megapixels, then there really are approximately 2,100,000 photosites on the CCD.

What happens is that some of the photosites are not being used for imaging. Remember that the CCD is an analog device. It's necessary to provide some circuitry to the photosites so that the ADC can measure the amount of charge. This circuitry is dyed black so that it doesn't absorb any light and distort the image.

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Is it best to buy still and movie digital cameras separately? What am I gaining or giving up?
If you don't have either a digital camera or a digital camcorder then you might be thinking about buying a camcorder that can also take still shots. There are couple things to think about before you buy this feature.

  • Generally, camcorders are bigger than digital cameras, so you have to consider whether the size of the camcorder will prevent you from taking it where you would normally take your camera.

  • Convenience of downloading the still pictures is important. Some camcorders store the still pictures on removable Flash memory cards, others store them right on the video tape. The Flash memory cards make transferring the pictures to a PC just as easy as any other digital camera. But the process of transferring pictures from the tape to a PC can be tedious.

  • Camcorders generally are not capable of taking pictures at resolutions as high as many digital cameras. This may be fine if you just want to e-mail pictures or post them on the Web, but if you want to be able to print high quality images a digital camera may be the better option.

  • The cost to add this feature to your camcorder may exceed the cost of a comparable digital camera.

  • If you just want to be able to shoot short movies (less than a minute or so) to post on the Web, many digital cameras can create movies in MPEG format.

Cool Facts

  • In the United States, there is roughly one camera for every adult.

  • With a 3-megapixel camera, you can take a higher-resolution picture than most computer monitors can display.

  • You can use your Web browser to view digital pictures taken using the JPEG format.

  • The first consumer oriented digital cameras were sold by Kodak and Apple in 1994.

  • In 1998, Sony inadvertently sold over 700,000 camcorders with a limited ability to see through clothes.

  • You can use various software programs to "stitch" together a series of digital pictures to create a large panorama.


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About the Author
Gerald Jay Gurevich is a writer in Chapel Hill, N.C.


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