Here is some info for digital cameras from
www.howthingswork.com that may be of interest.
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.
Lens and Focal Length
A camera lens collects the available light and focuses it on the sensor. Most digital cameras use automatic focusing techniques.
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.