Tuesday, March 4, 2008

More on photography

Depth of Field
Many people do not understand depth of field. Simply it is the region, in a picture that is acceptably sharp. What is acceptable is entirely dependant on how big an image is to be reproduced and how far away the viewer is. A picture, of a landscape - for example, that has sharp focus in the middle distance will be acceptably sharp from close-up to the far distance when printed at a small print size (6inch x 4inch) but will only be acceptably sharp at the point of focus at poster size (30inch x 40inch). This you can see for yourself by experiment using your computer printer -for instance.
Depth of field is governed by several things.
  1. Aperture: if you have control over your camera and can control the aperture size. The aperture is the iris diaphragm inside the lens system. When it is very small it is said to be stopped down (f8 say) and when it is wide open the aperture will be close to the maximum possible. Sometimes the maximum varies, with some makes and types of lens, the reason we dont need at this moment. When the lens is set wide open the depth of field, at a given focus point, will be shallower than when stopped down the smaller the aperture the more depth of field you get.
  2. Focal lenght: most cameras seem to have zoom capability. If this is through changes to the lens and not digital zoom the depth of field at wider angles of view will be greater than with narrower angles as when you zoom-in.
If you want to use selective depth of field, concentrating the attention to a small depthof the image, then you need to set the camera away from the subject and select a wide aperture ie F2.8 or f3.5. You will have the narrowest band of sharp focus at the maximum zoom and maximum aperture and corresponding greater sharpness at smaller amounts of zoom and/or aperture.
In the accompanying photograph I have photographed som rulers indicating the f stop number on each of three images. I will tell you more in my next episode.

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