When focus is secured on a certain distance the exact camera extension can be recorded by a mark on some convenient portion of the apparatus, so that the camera may be again set to the same mark when the same distance is to be focussed upon. A series of such marks forms a focussing scale, but it is not usually possible to fix all the marks by trial, and some have to be determined by calculation. It is easy to find the mark for an infinite distance by focussing on a very distant object, and also one or two marks for near objects at extra-focal distances of from 5 to 10 ft. from these marks; others can then be found by the following rules :—The distances represented by any two marks on the I scale are inversely proportional to the respective distances Of those marks from the infinity mark. Thus, if the infinity mark and the 5 ft. mark are both determined a mark halfway between the two represents 10 ft., and one halfway between the infinity and 10 ft. mark represents 20 ft. A number of marks may thus be set out by dividing the space from the infinity mark to that for 5 ft. by different factors.
The actual extension of the camera beyond the infinity mark is always equal to the square of the focal length divided by the extra-focal distance of the object in focus, all dimensions being in inches, and by this rule we can set out marks for any distances when the infinity mark alone is known.
The most convenient series of distances to show is the one of " consecutive depths " before described. The infinity mark is, of course, included in this series, the scale marks for which are all equal distances apart. Allowing for a circle of confusion of 1/100 in., each division of the scale is equal to 1/100 in. multiplied by the ratio number of the stop for which the series is calculated. Thus, with a series of consecutive depths for f/8, each division is -08 in.