When taking a photograph, there are really only three controls that matter on a camera: focus, shutter speed and aperture. Focus is reasonably obvious; you can quickly spot when a picture is out of focus. Likewise shutter speed is easy to understand since it is measured in fractions of a second. However aperture is a mystery to most people, because it is usually explained using technical jargon and its effects are not immediately obvious.
In this tutorial I'll attempt to explain how changing the aperture affects how your photograph will look, and how you can use those effects creatively to take better pictures.
The aperture is literally that; a hole through which light passes after it enters the lens. The size of this hole can be altered, allowing a greater or smaller amount of light to pass through.
Aperture is used in conjunction with shutter speed to control exposure, as explained in last month's tutorial. However it is also the primary means of controlling something called depth of field.
Depth Of Field
If you take a photo of someone at a range of about three metres with a normal automatic compact camera, in good light with the lens zoomed in about half way, you'll usually find that objects about 1.5m in front of the subject, and for about four or five metres behind the subject, also appear sharp. This distance, from the closest point of acceptable sharpness to the most distant, is the depth of field. By altering the size of the aperture, we can control the size of this depth of field, either reducing it so that only our subject is in focus, or expanding it so that an entire landscape can appear sharp.
If you have an older camera lying about, take a look at the lens. It should have a movable ring for controlling the aperture setting, labelled with numbers from about F2 to F22. It will also have a ring for adjusting the focus distance, usually calibrated in feet and metres. Alongside that scale, you'll usually find lines marked with the same numbers as the aperture ring, but in pairs either side of the distance indicator mark, with the larger F-numbers toward the outside. This is done to help estimate the depth of field at a particular distance and aperture setting; on the distance scale everything between the two lines for a given aperture setting will be acceptably sharp.
Unfortunately this feature is missing from most modern auto-focus, auto-aperture lenses.