The ASM functions give you a lot of control over your camera.
Most digital cameras have a set of easy modes to help you quickly find the settings you need for the photos you are about to take. However by using some of the more advanced settings---such as the aperture priority ("A"), shutter priority ("S") and manual ("M") options---photographers have even more control over their work.
Understanding Your Camera
What we think of as a camera is actually two main pieces of equipment working together: the SLR body and the lens.
The body takes the photograph when the shutter opens to allow light to the sensor.
Lenses help change the view of what we see with our eyes. They can also control how much light is received by the camera body through use of a diaphragm, called the aperture.
Aperture Priority
By selecting aperture priority, you determine how much light enters the camera---the lower the number, the more light. The "A" setting lets you choose the f-number for the camera, and it will automatically select the appropriate shutter speed.
The side effect of that is to change the depth-of-field of the image--- that is, how much of the picture is in focus and how much is blurred. A higher f-number will keep more of the image in focus; however, it also reduces the amount of light entering the camera, so your shutter speed will be slower.
Shutter Priority
By choosing the shutter priority option, you decide how long the shutter is open. Your camera then selects the ideal aperture setting. The "S" setting is useful to prevent excessive motion blur when shooting fast action, but you may not be able to use fast shutter speeds in low-light situations.
Manual Setting
You set both the shutter speed and the aperture setting when you select the "manual" setting. That setting should be used by more experienced photographers, as it involves understanding the light conditions and the limitations of your camera and lens. With practice, you will be able to strike the balance of shutter speed and aperture to make the most of available light.
Considerations
The lens is typically more important than the camera body when taking a photograph. Having an SLR camera that allows you to change lenses gives you more flexibility. Having lenses with the widest range of aperture settings will give you the most flexibility.
It is important to consider your ability to keep the camera still when choosing a shutter speed. Speeds faster than 1/160th of a second may be difficult to hand hold, but are easy to obtain with a tripod.
Tags: shutter speed, aperture priority, aperture setting, camera body, choosing shutter, control over