Understand how the depth of field works
Understand how the depth of field works | ||
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To understand why depth of field changes you need to remember how a lens critically focuses an image point at one distance only, depending on how far the lens is from the subject. In this position light from other parts of the subject nearer or farther from the lens comes to focus farther away or nearer, forming discs instead of points of light. They are known as circles of confusion. Large circles of confusion, overlapping give the image an extremely unsharp appearance. Since your eyes have limited resolving power, when viewing a final print or slide you rate an image acceptably sharp even when tiny discs are present instead of dots. The upper limit to what most people accept as sharp is taken to be 0.25 mm diameter on the final print. Lens manufacturers for 35 mm format cameras assume that if 25 × 20 cm (10 × 8 in) enlargements are made (film image magnified × 8) to this standard then the largest acceptable circle of confusion on film is 0.25 ÷ 8 = 0.03 mm. By accepting discs up to this size as sharp, subjects slightly nearer and farther away than the subject actually in focus start to look in focus too. And if the lens aperture is made smaller all the cones of light become narrower, so that images of subjects even nearer and farther are brought into the zone of acceptable sharp focus. Depth of field has increased. Again if you move farther back from the subject or change to a shorter focal length lens, the positions of sharp focus for images of nearest and farthest subject parts bunch closer together. Their circles of confusion become smaller, again improving depth of field. So remember that you produce greatest depth of field when:
With subjects beyond about ten focal lengths from the lens, depth of field extends farther behind the subject than towards the lens. Hence the photographer's maxim 'focus one-third in', meaning focus on part of the scene one-third inside the depth of field required. With close-up work, however, depth of field extends more equally before and behind the focused subject distance. | ||