This is an important concept when considering how to focus a lens to achieve maximum depth of field for a particular scene. When photographing a landscape, for example, photographers who do not understand hyperfocal distance are likely to select the smallest available aperture and focus at infinity. However, this approach does not result in maximum depth of field.
Hyperfocal distance is the the shortest distance from a lens to an in-focus subject, for a particular aperture, when the lens is focused at infinity. When set to the hyperfocal distance depth of field extends from about half the hyperfocal distance to infinity. The distance is dependent upon the focal length of the lens, the aperture selected and the diameter of the circle of confusion. Hyperfocal distance is calculated using the equation:
H = {F2 / (A x C) } + F
where H is the hyperfocal distance, F is the focal length of the lens, A is the aperture and C the diameter of the circle of confusion (typically about 0.03mm for the 35mm format). Note that all variables must be stated using the same units (eg mms). For a typical (35mm format) 50mm lens set to f/22, and using the typical value for the circle of confusion, the hyperfocal distance is approximately 3.75 metres. In this example, if the lens were therefore focused at 3.75 metres, acceptable depth of field would extend from approximately 1.9 metres to infinity.
The nearest distance of acceptable sharpness is calculated using the equation:
N = D (H - f) / (H + D - 2f)
where N is the nearest distance of acceptable sharpness, D is the focused distance, H is the hyperfocal distance (above), and f is the focal length of the lens.
The furthest distance of acceptable sharpness is given by:
R = D (H - f) / (H - D)
where R is the furthest distance of acceptable sharpness, D is the focused distance, H is the hyperfocal distance (above), and f is the focal length of the lens.