The in depth definition of Depth Of Field

Depth of field can be defined as the distance between the furthest and nearest points of a subject that are acceptably sharp. When you focus on a particular point of a subject there is a zone of acceptable sharpness beyond which the subject detail is unsharp. In 'normal' photography the zone of acceptable sharpness extends from one-third in front of the main point of focus, to two-thirds behind it. A good rule of thumb to maximize depth of field is to focus one-third into the subject. This is true for 'normal' photography of distant subjects, but as the magnification of the subject increases, these relative proportions change. Therefore, at a magnification of 1×, the distances in front of and behind the main point of focus become equal, and you should focus on a point halfway into the subject.

The key word in the above definition is 'acceptable,' and this is obviously subjective—different photographers may well have different opinions as to what is acceptably sharp. Various formulas have been devised to calculate depth of field for different magnification/aperture combinations, and make use of the concept of 'circles of confusion' to define sharpness, and take into account the assumed final size of reproduction.

Depth of field is dependent on magnification and aperture, not focal length. If the same subject is photographed with a 50 mm lens and then a 200 mm lens, and the distance from the camera to the subject is altered so that the magnification of the subject is the same in each case, and the same aperture is used in each case, then the depth of field will be the same. What will vary is the perspective of the image, and how much of the background behind the subject can be seen. What a longer focal-length lens will give is a greater working distance between the subject and the front of the lens, minimizing the risk of disturbance to a small insect, for example, or giving sufficient space to illuminate the subject with flash.

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