In the past twenty years, most of the major technological breakthroughs in consumer electronics have really been part of one larger breakthrough. When you get down to it, CDs, DVDs, HDTV, MP3s and DVRs are all built around the same basic process: converting conventional analog information (represented by a fluctuating wave) into digital information (represented by ones and zeros, or bits). This fundamental shift in technology totally changed how we handle visual and audio information -- it completely redefined what is possible.

The digital camera is one of the most remarkable instances of this shift because it is so truly different from its predecessor. Conventional cameras depend entirely on chemical and mechanical processes -- you don't even need electricity to operate them. On the other hand, all digital cameras have a built-in computer, and all of them record images electronically.

The new approach has been enormously successful. Since film still provides better picture quality, digital cameras have not completely replaced conventional cameras. But, as digital imaging technology has improved, digital cameras have rapidly become more popular.

In this article, we'll find out exactly what's going on inside these amazing digital-age devices.

Understanding the Basics
Let's say you want to take a picture and e-mail it to a friend. To do this, you need the image to be represented in the language that computers recognize -- bits and bytes. Essentially, a digital image is just a long string of 1s and 0s that represent all the tiny colored dots -- or pixels -- that collectively make up the image. (For information on sampling and digital representations of data, see this explanation of the digitization of sound waves. Digitizing light waves works in a similar way.)

If you want to get a picture into this form, you have two options:

• You can take a photograph using a conventional film camera, process the film chemically, print it onto photographic paper and then use a digital scanner to sample the print (record the pattern of light as a series of pixel values).

• You can directly sample the original light that bounces off your subject, immediately breaking that light pattern down into a series of pixel values -- in other words, you can use a digital camera.

At its most basic level, this is all there is to a digital camera. Just like a conventional camera, it has a series of lenses that focus light to create an image of a scene. But instead of focusing this light onto a piece of film, it focuses it onto a semiconductor device that records light electronically. A computer then breaks this electronic information down into digital data. All the fun and interesting features of digital cameras come as a direct result of this process.

In the next few sections, we'll find out exactly how the camera does all this.

Cool Facts

• 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 more than 700,000 camcorders with a limited ability to see through clothes.

A Filmless Camera

A CMOS image sensor

Instead of film, a digital camera has a sensor that converts light into electrical charges.

The image sensor employed by most digital cameras is a charge coupled device (CCD). Some cameras use complementary metal oxide semiconductor (CMOS) technology instead. Both CCD and CMOS image sensors convert light into electrons. If you've read How Solar Cells Work, you already understand one of the pieces of technology used to perform the conversion. A simplified way to think about these sensors is to think of a 2-D array of thousands or millions of tiny solar cells.

Once the sensor converts the light into electrons, it reads the value (accumulated charge) of each cell in the image. This is where the differences between the two main sensor types kick in:

• A CCD transports the charge across the chip and reads it at one corner of the array. An analog-to-digital converter (ADC) then turns each pixel's value into a digital value by measuring the amount of charge at each photosite and converting that measurement to binary form.

• CMOS devices use several transistors at each pixel to amplify and move the charge using more traditional wires. The CMOS signal is digital, so it needs no ADC.

Photons hitting a photosite and releasing electrons

Differences between the two types of sensors lead to a number of pros and cons:

Photo courtesy DALSA A CCD sensor

• CCD sensors create high-quality, low-noise images. CMOS sensors are generally 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 hit the transistors instead of the photodiode.
• CMOS sensors traditionally consume little power. CCDs, on the other hand, use a process that consumes lots of power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.
• 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.

Although numerous differences exist between the two sensors, they both play the same role in the camera -- they turn light into electricity. For the purpose of understanding how a digital camera works, you can think of them as nearly identical devices.

Digital Camera Resolution

The amount of detail that the camera can capture is called the resolution, and it is measured in pixels. The more pixels a camera has, the more detail it can capture and the larger pictures can be without becoming blurry or "grainy."

Some typical resolutions include:

• 256x256 - Found on very cheap cameras, this resolution is so low that the picture quality is almost always unacceptable. This is 65,000 total pixels.
• 640x480 - This is the low end on most "real" cameras. This resolution is ideal for e-mailing pictures or posting pictures on a Web site.
• 1216x912 - This is a "megapixel" image size -- 1,109,000 total pixels -- good for printing pictures.
• 1600x1200 - With almost 2 million total pixels, this is "high resolution." You can print a 4x5 inch print taken at this resolution with the same quality that you would get from a photo lab.
• 2240x1680 - Found on 4 megapixel cameras -- the current standard -- this allows even larger printed photos, with good quality for prints up to 16x20 inches.
• 4064x2704 - A top-of-the-line digital camera with 11.1 megapixels takes pictures at this resolution. At this setting, you can create 13.5x9 inch prints with no loss of picture quality.

Photo courtesy Morguefile The size of an image taken at different resolutions

High-end consumer cameras can capture over 12 million pixels. Some professional cameras support over 16 million pixels, or 20 million pixels for large-format cameras. For comparison, Hewlett Packard estimates that the quality of 35mm film is about 20 million pixels [ref].

Next, we'll look at how the camera adds color to these images.

How Many Pixels? You may have noticed that the number of pixels and the maximum resolution don't quite compute. For example, a 2.1-megapixel camera can produce images with a resolution of 1600x1200, or 1,920,000 pixels. But "2.1 megapixel" means there should be at least 2,100,000 pixels.

This isn't an error from rounding off or binary mathematical trickery. There is a real discrepancy between these numbers because the CCD has to include circuitry for the ADC to measure the charge. This circuitry is dyed black so that it doesn't absorb light and distort the image.

Capturing 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 the camera records all three colors, it combines them to create the full spectrum.

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. A beam splitter directs light to the different sensors. Think of the light entering the camera as water flowing through a pipe. Using 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, each sensor only responds to one of the primary colors.

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 tend to be bulky and expensive.

Another 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.

A spinning disk filter

Both of these methods work well for professional studio cameras, but they're not necessarily practical for casual snapshots. Next, we'll look at filtering methods that are more suited to small, efficient cameras.

More on Capturing Color

A more economical and practical way to record the primary colors is to permanently place a filter called a color filter array 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.

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. The pixels are not evenly divided -- 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."

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. The raw output from a sensor with a Bayer filter is a mosaic of red, green and blue pixels of different intensity.

Digital cameras use specialized demosaicing algorithms to convert this mosaic 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.

A demosaicing algorithm at work

Some single-sensor cameras use alternatives to the Bayer filter pattern. X3 technology, for example, embeds red, green and blue photodetectors in silicon. Some of the more advanced cameras subtract values using the typesetting colors cyan, yellow, green and magenta instead of blending red, green and blue. There is even a method that uses two sensors. However, most consumer cameras on the market today use a single sensor with alternating rows of green/red and green/blue filters.

Exposure and Focus

Just as with film, a digital camera has to control the amount of light that reaches the sensor. The two components it uses to do this, the aperture and shutter speed, are also present on conventional cameras.

• Aperture: The size of the opening in the camera. The aperture is automatic in most digital cameras, but some allow manual adjustment to give professionals and hobbyists more control over the final image.
• Shutter speed: The amount of time that light can pass through the aperture. Unlike film, the light sensor in a digital camera can be reset electronically, so digital cameras have a digital shutter rather than a mechanical shutter.

These two aspects work together to capture the amount of light needed to make a good image. In photographic terms, they set the exposure of the sensor. You can learn more about a camera's aperture and shutter speed in How Cameras Work.

In addition to controlling the amount of light, the camera has to adjust the lenses to control how the light is focused on the sensor. In general, the lenses on digital cameras are very similar to conventional camera lenses -- some digital cameras can even use conventional lenses. Most use automatic focusing techniques, which you can learn more about in the article How Autofocus Cameras Work.

The focal length, however, is one important difference between the lens of a digital camera and the lens of a 35mm camera. The focal length is the distance between the lens and the surface of the sensor. Sensors from different manufacturers vary widely in size, but in general they're smaller than a piece of 35mm film. In order to project the image onto a smaller sensor, the focal length is shortened by the same proportion. For additional information on sensor sizes and comparisons to 35mm film, you can visit the Photo.net Web site.

Focal length also determines the magnification, or zoom, when you look through the camera. In 35mm cameras, a 50mm lens gives a natural view of the subject. Increasing the focal length increases the magnification, and objects appear to get closer. The reverse happens when decreasing the focal length. A zoom lens is any lens that has an adjustable focal length, and digital cameras can have optical or digital zoom -- some have both. Some cameras also have macro focusing capability, meaning that the camera can take pictures from very close to the subject.

Digital cameras have one of four types of lenses:

• Fixed-focus, fixed-zoom lenses - These are the kinds of lenses 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, these have "wide" and "telephoto" options and automatic focus. The camera may or may not support manual focus. These actually change the focal length of the lens rather than just magnifying the information that hits the sensor.
• Digital-zoom lenses - With digital zoom, the camera takes pixels from the center of the image sensor and interpolates them to make a full-sized image. Depending on the resolution of the image and the sensor, this approach may create a grainy or fuzzy image. You can manually do the same thing with image processing software -- simply snap a picture, cut out the center and magnify it.
• Replaceable lens systems - These are similar to the replaceable lenses on a 35mm camera. Some digital cameras can use 35mm camera lenses.

Next, we'll learn about how the camera stores pictures and transfers them to a computer.

Storing Digital Photos

Most digital cameras have an LCD screen, so 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. Of course, 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 do this.

Photo courtesy HSW Shopper A CompactFlash card

Early generations of digital cameras had fixed storage inside the camera. You needed to connect the camera directly to a computer with cables to transfer the images. Although most of today's cameras are capable of connecting through serial, parallel, SCSI, USB or FireWire connections, they usually also use some sort of removable storage device.

Digital cameras use a number of storage systems. These are like reusable, digital film, and they use a caddy or card reader to transfer the data to a computer. Many involve fixed or removable flash memory. Digital camera manufacturers often develop their own proprietary flash memory devices, including SmartMedia cards, CompactFlash cards and Memory Sticks. Some other removable storage devices include:

• Floppy disks
• Hard disks, or microdrives
• Writeable CDs and DVDs

No matter what type of storage they use, all digital cameras need lots of room for pictures. They usually store images in one of two formats -- TIFF, which is uncompressed, and JPEG, which is compressed, but some use RAW format. Most cameras use the JPEG file format for storing pictures, and they sometimes offer quality settings (such as medium or high). The following chart will give you an idea of the file sizes you might expect with different picture sizes.

Image Size	TIFF (uncompressed)	JPEG (high quality)	JPEG (medium quality)
640x480	1.0 MB	300 KB	90 KB
800x600	1.5 MB	500 KB	130 KB
1024x768	2.5 MB	800 KB	200 KB
1600x1200	6.0 MB	1.7 MB	420 KB

To make the most of their storage space, almost all digital cameras use some sort of data compression to make the files smaller. Two features of digital images make compression possible. One is repetition. The other is irrelevancy.

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. 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 the human eye can easily detect. Some compression routines take advantage of this fact to throw away some of the more meaningless data.

Next, we'll tie it all together and see how a digital camera takes a picture.

Summary

It takes several steps for a digital camera to take a picture. Here's a review of what happens in a CCD camera, from beginning to end:

• You aim the camera at the subject and adjust the optical zoom to get closer or farther away.

• You 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.

• You press the shutter release all the way.

• The camera resets the CCD and exposes it 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 in some form of memory device (probably a Flash memory card).

Photo courtesy HSW Shopper
A memory stick

With a digital camera, you can also take as many pictures as you want without worrying about wasting film. You can look at pictures right away and decide whether to keep them or delete them. And, you can print only the images you want -- you don't have to process whole rolls of film and then figure out where to store all the pictures.

But digital cameras also have a few disadvantages. A film camera can take a picture almost immediately when you press the button. Digital cameras, on the other hand, can take a few seconds, especially if they're making adjustments automatically. They also require more light than traditional cameras do. Sometimes, the abundant space on a memory card encourages people to take so many pictures that they're not sure what to do with them later.

By keeping a few tips in mind, it's easy to overcome the disadvantages and get a lot out of your digital camera. The first step is to know the basics of how camera works. A digital camera is a lot like a film camera, but it uses a sensor and a memory card in place of film. When you take a picture, a digital shutter opens and exposes the sensor to light. The sensor reacts to the light, and the memory card stores the resulting picture.

The light that hits the sensor determines virtually everything about the picture. Your camera may be able make some adjustments, but it has to work with the amount and type of light that it receives. You can control how the light hits the sensor with three settings:

• The focus adjusts the lens in relation to the sensor, making sure that the light converges on the sensor's surface. Most digital cameras have an automatic focus feature, but a few focus manually and have interchangeable lenses.
• The aperture, measured in f-stops, determines how wide the shutter opens. A wide opening lets in more light, and a narrow opening lets in less. Higher f-stops mean that the opening is smaller, and lower f-stops mean that it is larger. The aperture determines the picture's depth of field, or how much of the picture is in focus. At low f-stops, the foreground of the picture will be in focus while the background is out of focus. At high f-stops, objects in both the foreground and background are in focus.
• The shutter speed determines how long the shutter is open. The longer it's open, the more light will hit the sensor. If you or your subject is moving while the shutter is open, the image will be blurred.

On most cameras, you can manually adjust the aperture and shutter speed. This can reduce the amount of time between when you press the button and when the shutter opens. Most cameras also have a fully automatic mode that adjusts the settings, including the focus, for you. You can take lots of good, clear pictures with this mode. Many cameras also have several presets for capturing portraits, fast action or outdoor scenes.

Most cameras have a selection dial or another means of changing its settings.

But automatic exposure might not be best for every situation, so understanding f-stop, shutter speed and presets can make a huge difference in how well your camera works for you. Being able to set the exposure on your own will also let you cut down the amount of time between when you press the button and when the shutter opens. In other words, you can take the picture you want before the moment passes. You can learn more about setting exposure by taking several pictures of the same scene using different settings and seeing how the finished product changes.

Once you have a feel for the settings, there are a few other steps you can take to make get more from your camera:

• Hold the camera steady. Since digital cameras require more light than film cameras, the shutter is often open longer. This can cause your pictures to blur. Using a tripod or monopod can help you keep your camera still.

• Lock the focus. On most auto focus digital cameras, pressing the button halfway will focus the camera. You can hold the button in this position until you're ready to take the picture. This can further reduce the time between when you press the button and when the shutter opens. It can also let you keep the camera in focus while re-framing the picture.

• Use optical -- not digital -- zoom. An optical zoom physically changes how far the lens is from the sensor. Digital zoom, on the other hand, simply forces the camera to create the picture from one portion of the sensor rather than the whole thing. You can do the work of a digital zoom yourself using image editing software, and you can often do it better than your camera can.

• Preserve the battery. It's tempting to use the LCD screen as a viewfinder. Sometimes, it's the only good way to see what you're taking a picture of. But the LCD screen uses lots of battery power. If possible, set your camera to preview pictures on the screen after you take them but to keep it turned off the rest of the time.

• Delete unwanted pictures right away. Unless you're quickly taking several pictures of the same scene, look at your picture as soon as you take it. You'll know right away if you need to take another. If you do, go ahead and delete the one you don't like. If you wait to review all your pictures and delete unwanted ones, your camera will probably insert new pictures into the spaces the deleted ones left. This can make it harder to sort and organize your pictures later.

• Maximize your storage space. Most cameras come with a very small memory card. Upgrade it to something larger, and keep the old one as an emergency backup. You can fit more pictures onto your card by lowering the resolution or increasing the compression that the camera uses. Even if your camera has a high megapixel rating, you can manually set it to take slightly lower-quality pictures. You should still be able to make average-sized prints with little to no loss of quality.

Image courtesy Morguefile A camera's resolution, measured in megapixels, affects picture detail and the maximum size of prints.

But taking pictures is only the first step in using a digital camera. Next, we'll look at how to organize, print and share your pictures.

Organizing, Printing and Sharing

After taking pictures with your digital camera, the next step is to transfer them from the memory card to your computer. Depending on your camera, you either remove the card and place it in a card reader, or you plug your camera directly into your computer's USB or FireWire port. Some people wait until the memory card is nearly full, but it's a good idea to go ahead and copy your pictures soon after you take them. That way, you won't lose your pictures if something happens to your camera, and you can sort and share your pictures while they're still fresh on your mind. You should also back your pictures up onto CD-ROM or DVD-ROM regularly.

Many digital cameras come with software to help you manage your pictures. Often, the program will show you thumbnails, or miniature versions of your pictures, so you can quickly navigate through your collection. You can use the program to categorize your photos by date, subject matter or type. Many programs also let you make basic image corrections, like removing red eye. Or, you can use more advanced software, like Adobe Photoshop, to completely change the look of your pictures. See Digital Photography Basics for more details.

Some digital cameras come with software, such as Kodak's EasyShare.

After transferring pictures, some people get stuck -- they wind up with a computer full of images that they're not quite sure what to do with. But there are lots of options for sharing and displaying your pictures. You can print them, upload them to an image sharing site, e-mail them or incorporate them into a Web page.

You can print pictures at home on a photo printer, or you can send them to a printing service. Both options have some pros and cons. If you buy a printer, you can print your pictures right away. Many printers are portable, so you can take them with you on vacation and print photos as you go. But the cost of the printer, ink and paper can add up -- in general, this is an expensive way to print your photos.

Image courtesy Amazon

Most of the time, using a professional printing service is a more economical option. You can make standard-sized prints for a few cents each and very large prints for a few dollars. Typically, you sign up for a free account, upload the photos you want to print, and place your order. Sometimes, you can take CDs of images to an in-store kiosk and place your order there.

Using a professional service is generally cheaper than buying your own printer, and your prints will probably be better quality. However, uploading the pictures can take time, since you'll want to send full-sized, high-resolution pictures. You'll also have to wait a few days for your prints to arrive.

If you're going to share your photos via e-mail, it's a good idea to re-size them before you send them. Digital cameras can create very large image files, and these files can take a long time to download. To reduce the size of your pictures, open them in your photo editing software. Make the files smaller in one of two ways:

• Reduce the resolution, measured in dots per inch (DPI). The minimum resolution for good-quality onscreen viewing is 72 DPI.
• Reduce the image size, measured in inches, centimeters or pixels. A 3 by 5 inch (7.6 by 12.7 centimeter) picture will travel well by e-mail but will still be big enough to see.

Save the result as a new file, and keep your original high-resolution image.

If you'd rather share your photos in an online gallery than via e-mail, you can start an account on an image-sharing site like Flickr, Fotki or Snapfish. Your camera's software may also include automatic uploading options. Many of these services are free, but some limit how many pictures you can upload per month or how much disc space you can use. CNET has a good comparison of online sharing and printing sites.

Finally, some people start taking lots of pictures after buying a digital camera -- so many that sorting them and choosing which ones to print becomes a chore. Rather than taking hundreds of pictures in the hope of getting a few good ones, it's a good idea to learn a little about color, light and composition.