Many photographers find it difficult to keep track of depth of field as they work. They move around a subject and the focused distance changes. They may also alter the aperture and focal length of the lens. Depth of field scales marked on lenses certainly help, but reading them interrupts the flow of work. Preview facilities, that close lenses down to chosen apertures, are intended to make depth of field visible through the viewfinder but consequently reduce the brightness of the eyepiece image. Some people can cope with these things – others cannot  One possible solution is to use reproduction ratio.

This is a very simple but little-used measurement. The eyes of some first-class photographers glaze over at the very mention of the term. This is surprising because even rudimentary understanding is useful in a practical way.

Depth of field depends principally upon three variables - aperture, focal length and focused distance. It is nevertheless beyond most of us to do the mental arithmetic on the fly. Fortunately two of the variables, focal length and focused distance, combine to give reproduction ration. This is the ratio of the size of the subject’s image on film to the actual physical dimensions of the subject, a product quantified simply by observing how large the subject appears in the viewfinder.

Consider an example. If a person's head comfortably fills a landscape-format 35mm frame, using any lens, the image is about 24mm in height. If we assume for convenience that a human head is actually 288mm in height, the reproduction ratio is 24:288 or 1:12. Since lens manufacturers quote depth of field figures for given reproduction ratios and apertures, all the brain has to do is track aperture and memorize the corresponding depth of field.

There is nothing clever about this. When working close to a subject just remember, for instance, that when a person's head fills a horizontal 35mm frame the reproduction ratio is 1:12. At f/5.6 this gives depth of field of about five centimetres. It is as simple as that - forget about lenses, zooming and distances.