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Aperture - the basics
To understand how aperture works on a camera it helps to compare it to the pupil of the human eye. The less light there is, the wider you open your pupil, while if there is a lot of light it narrows down to a small opening and blocks the amount of incoming light. The aperture on a camera, which is measured in f-stops, does exactly the same thing by controlling the amount of light that reaches the CCD. Lower f-stop numbers (e.g. 2.8) widen the aperture and allow more light to get to the CCD, while higher f-stop numbers (16 or 22) limit the amount of light by making the camera?s aperture smaller. It should be noted that if the aperture is opened up by one stop, the amount of light reaching the CCD is doubled. At a given exposure value, aperture and shutter speed are always interdependent. If you change the former, the latter will have to be changed as well to keep exposure the same.
Aperture openings also control depth of field. Smaller aperture openings keep a larger part of the image in sharp focus while larger aperture openings will keep the subject or focal point pin-sharp while the rest of the image remains blurred. This effect is even more obvious with tele lenses as they have smaller depths of field than wide- angles.
Aperture - the pitfalls
Beware that when we talk about apertures high numbers (16 or 22) indicate small openings and low numbers (2,8 or 4) mean large openings.
The smaller the aperture, the larger depth of field. However, you should know that DOF extends 1/3 in front of the point of focus and 2/3 beyond it. So by focusing at infinity, you are in fact wasting 2/3 of your depth of field. Focusing at a point in front of infinity and choosing a small aperture will maximize DOF. This is called hyperfocal focusing.
If your camera has a depth of field scale, here is how to do it. Focus your camera at infinity, note the distance on the camera?s lens opposite the chosen aperture and set your lens at this hyperfocal distance. Now everything from foreground to infinity will be sharp.
It is best not to choose the widest or smallest aperture setting because lenses do not show their optimum image quality at both extremes of the scale and optical faults may become visible. For optimum image quality select a medium aperture.
In low light situations, fully automatic cameras give preference to fast shutter speeds and select the widest aperture setting for a correct exposure, so depth of field will be minimal.
If there is no manual way to select a slower shutter speed keep your subject away from the borders of your image as this is where optical faults are most prominent.
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