Proper exposure is curcial, without it you won't get an image--too much or too little exposure is the main reason for digital images to come out poorly or not at all.
Proper exposure is established by the camera through the use of a light meter and a certain combination of mechanical settings which control the amount of light that reaches the sensor. These are known as the Shutter and the Aperture.
The camera is mainly a box that holds the light sensitive sensor. It also has a lens that focuses light onto the sensor. The most crucial part of taking a photograph is controlling how much light reaches the sensor so that it will in fact create an image. The camera has two devices that control the exposure of the snesor:
The shutter in the camera can be manipulated to allow the light to reach the film for only a split second, or many seconds long. The most common shutter speeds are fractions of a second such as 1/60th of a second or faster; up to 2000/th of a second on average. These fractions of a second are denoted on your camera simply with denomenator. (i.e. 1/60th, as simply 60).
The aperture in the lens can control how much light reaches the shutter. It works in the same basic way that the Iris does in our eyes. By constricting the aperture to a small hole, the lens will only allow a fraction of the light that it normally does. The normal aperture settings correspond to exactly half or twice as much light as the previous setting.
Why use specific settings?
Because the shutter can reduce your exposure time to a fraction of a second, it can be used to "freeze" action such as sports events or moving water. The shutter can also be used at slow settings (long shutter openings) to allow you to shoot in dim lighting, and to allow you to blur your subject if you want--such as the "milky" rivers in some landscapes. This bluring action happens when the movement of the subject takes place at a faster pace than the shutter speed. The rule of thumb is: don't try to shoot a photo holding the camera at a speed slower than 1/60th of a second, or you'll get camera movement and your entire photo will be blurry. For slow speeds it is recommended that you use a tripod to avoid such camera movement.
The aperture can reduce or increase the amount of light that actually gets through the lens, but it can also effect your picture's focus. When you look through your camera, you are generally looking through it at its widest aperture (so that you get a bright view of your scene). When you push the exposure button, the camera will quickly constrict the aperture to whatever setting the camera is on.
Large apertures (like the one that you see when you look through the viewfinder) represent the world a lot like we do with our eyes; whatever the lens is focused on will be sharply focused, but the background and foreground will be blurry. This is generally referred to as a shallow depth of field, or depth of focus. Small apertures on the other hand, when the aperture is constricted to a very small opening, have the effect on the appearance of the image that is referred to as large depth of field. It will give the appearance of having everything in focus; both background and foreground. This is not how we normally see the world and so this aspect of the photographic image is one that is unique. The depth of field of a photograph is a description of how much range of distance
is sharply focused in the image.
You probably have seen how many portraits only have the subject in focus and the background is out of focus. This effect is created with a wide or large aperture and the resulting narrow or shallow depth of field.
Large Depth of Field
In many landscape image, you probably have seen just the opposite, foreground, middleground and background all in sharp focus. This effect is achieved with a small aperture opening and results in a large deep depth of field range.
Very often, these landscape photos are shot with a tripod, because the small aperture severely cuts down the amount of light reaching the film, and thereby forces the photographer to use a slow shutter speed to compensate. In the same vein, photographs of fast action often need
to use a wide aperture to compensate for the very short interval of time the fast shutter will allow light to be recorded on the film