피사계 심도와 배율

Depth of Field and Magnification

Depth of field (DOF) means the range between the nearest and the farthest point of acceptable sharpness when the lens is focused at a certain distance. It is usually understood to be dependent upon three factors (in addition to the permissible size of circle of confusion): the distance s at which the lens is focused, the focal length f of the lens, and its f-stop N . While the three factors are the ones camera users usually encounter and have to decide upon when shooting, their relation to DOF is very complicated and often not intuitive. Especially, many people are confused about the relation of sensor size to DOF. In this paper the author explains DOF in terms of image magnification M and the actual aperture d =f /N . It is suggested that we think of, not the nearest and farthest geographical points of acceptable sharpness, but the magnifications, M N and M F, of images corresponding to those near and far points. We might coin a term, "depth of magnification", that denotes the difference or ratio between M N and M F. It is shown that such "depth" can be determined by only two factors : d and M S (magnification of the image of the subject at distance s).