Depth of Field Explained version 2

Sometimes you want to be ale to concentrate attention on your subject by having it sharp while the rest of the picture is out of focus and sometimes you want to be able to show the relationship of the subject with it's surroundings by having everything fairly sharp. You can do this to some extent by paying attention to depth of field when you are taking the picture.
When you focus a lens the subjects at further distances from the point at which you are actually focussed will graually become more and more blurred the further from the actual point of focus that they are. This is true for objects both further or nearer to the lens than the actual point of focus. The following factors affect the degree of bluring of the out of focus objects:-

Distance from the point of focus


This is distance from the lens, not distance from the subject. Two objects that are the same distance away from the camera will both be equally sharp (or equally unsharp) using normal lenses with no camera movements (no tilting of the lens from a parallel plane to the sensor).

Working aperture


If a lens has a maximum aperture of f2.8 and a minimum aperture of f16 then depth of field will be least at f2.8 and, as you stop down the lens, the depth of field will increase until the maximum depth of field will be reached at minimum aperture. These pictures were taken to illustrate the points. All pictures were taken at aproximately the same subject magnification. The top row are the whole image and the second row are a cropped version of the file.

Variation of depth of field with aperture.
Note how the background gats sharper the more you stop down (Left to right).

28mm lens f2.8 28mm lens f8 28mm lens f16
35mm lens f2 35mm lens f4 35mm lens f8 35mm lens f16
50mm lens f1.4 50mm lens f2 50mm lens f8 50mm lens f16
85mm lens f2 85mm lens f4 85mm lens f8 85mm lens f16
105mm lens f2.8 105mm lens f4 105mm lens f8 105mm lens f16

Variation of depth of field focal length of lens.
Note how the background is sharper with wide angle lenses at the same aperture. Notice also, how the perspective changes so that amount of background visible behind the model decreases with longer lenses.
14mm lens f16 28mm lens f16 35mm lens f16 50mm lens f16 85mm lens f16 105mm lens f16

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