The man who is opening a new studio or who thinks of buying a new lens for the old studio is often at a loss to determine the length of focus the lens should have to suit his needs best

Short focus and long focus lenses both have advantages, but, as a rule, the longest focus lens that can be used for the work it is required to do is the lens that will produce the most satisfactory results.

Tables are published to help in the selection of lenses, but such tables are sometimes confusing, and as the rule by which the calculations are made is not published the photographer can not very well work out his own problem.

A lens is often used for standing figures as well as bust portraits, but the calculation can be made so accurately that the lens of proper focal length for any working distance that may be convenient or practical for the photographer to use, may be determined. And it can also be determined at exactly what distance the lens used for full figures must be from the subject for a three-quarter length or head and shoulder image of any size.Such calculation saves the trouble of ordering a lens and finding, on trial, that it does not meet with requirements.The things you must know in selecting a lens are the greatest distance, lens to subject, at which you can work conveniently and the relative size of the subject to the image you wish to secure in your negative.

If your operating room is thirty feet long and your skylight is so arranged that you must place your subject six feet from the end wall to secure the proper lighting and allow sufficient space for your background, your working space is reduced to twenty-four feet. Allowing another six feet for your camera, and the operator behind the camera, reduces the actual working space to eighteen feet.

Suppose the average subject you photograph is five feet five inches tall and you wish to make a full figure picture having the image five inches high in your negative. To determine the focal length of the lens that will meet these conditions the working distance is divided by the number of diameters reduction, plus one. To find the number of diameters reduction, divide the height of subject by the height of the image you wish.

Height of subject, 65 inches, divided by height of image, 5 inches, equals 13, plus 1 equals 14. And working distance, 216 inches, divided by 14 equals 15 3/7 inches, which is the exact focal length the lens should be.

FROM AN ARTURA IRIS PRINT

By Clarence Stearns Rochester, Minn.

In round figures a 15 inch focus lens would come well within the requirements of conditions stated in this example. A 16 inch lens would require 18 feet 8 inches space and might crowd the operator a trifle. To find the working distance when the focal length is known, multiply the diameters reduction plus one by the focal length.

The 15 inch lens would be excellent for three-quarter figures and head and shoulder work but might not be so good for groups, considering the limited working space. If, however, the lens would cover a 6½ x 8½ plate, a group could be made in which the images would be the same size and the working distance the same as for single figures.

A lens suitable for groups may be selected by finding the width of the space the group will occupy under the skylight and dividing this by the width of space the image should occupy on a plate of any size. The result is the reduction in diameters, and the calculation can be made the same as for a single figure.

While this rule may not be useful to you at the time of reading it may come in handy at some later time and should be filed with similar information for future reference.

An Eastman Timer will repay its cost several times over in one month in the paper it saves from the waste basket.