Depth of field may be defined as the range of distances from the lens within which objects appear acceptably sharp. In broad terms this range extends one third before and two thirds beyond the point of focus - the only point at which the image is critically sharp. The diagram shows the range of distances from an 85mm lens over which acceptable sharpness is achieved at various apertures. Depth of field is nevertheless a subjective measurement. The concept arises from the inability of the eye to distinguish between a small blurred circle and a point within an image. It is also a product of the geometry of image formation, and hence something that no lens designer can eliminate.
A perfectly sharp image may be considered to be composed of pinpoints of light. However, sharpness falls off with increasing distance from the point of focus and each pinpoint of light blurs into a tiny circle. The diameter of the circle increases as the distance becomes larger. Most importantly, the larger the lens aperture the faster the blurred circle expands. Lens designers use this phenomenon to decide what may be considered sharp. Subjective measurements are made to determine the maximum diameter of the so-called circle of confusion before the image is no longer perceived as sharp. Depth-of-field measurements are then based upon a particular diameter for the circle of confusion - typically 1/30mm.
Depth of field is proportional to the diameter of the circle of confusion, the f-number and the square of the focused distance, and inversely proportional to the square of the focal length of the lens. In practical terms this means that depth of field increases as the aperture gets smaller (large f-numbers such as 16 or 22), and as distance from the subject increases. It decreases as the focal length of the lens increases, so in that sense telephoto lenses have less depth of field than wide-angle lenses.
However, it is important to bear in mind that the image obtained from a telephoto lens can be cut from the centre of an image taken from the same location with a wide-angle lens. When the centre of the wide-angle image is enlarged and compared with the telephoto view, depth of field will be same in both cases. Indeed, the only difference between the two images will arise from resolution. Film grain or pixelation will be more apparent in the image taken from the centre of the wide-angle view, and sharpness will consequently be less critical.
Depth of field for any lens and aperture combination, and many other parameters, can be obtained using the depth of field calculator.