Capturing Color: Beam Splitter
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. 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.

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

In the previous sections, 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.  

Think of each photosite as a bucket or a well, and 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. Sticking to the analogy, the ADC measures the depth of the water, which is considered analog information. Then it converts th

Photosites and 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. 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 at information to binary form.

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. 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.

Controlling Light
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 the next couple of sections, you will learn how the aperture and shutter work together to control the amount of light that enters the camera.

Aperture
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 functions common to film-based cameras.

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.

 

Batteries


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.

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

The important difference between the lens of a digital camera and the lens of a 35mm 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 35mm cameras, a 50mm 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 farther away, but you can capture a wider field of view in the camera.

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 manually do the same thing a digital zoom is doing -- simply snap a picture and then cut out the center of the image using your image processing software.
  • Replaceable lens systems - If you are familiar with high-end 35mm 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 35mm cameras in some cases

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