To those desiring advice on the choice of a lens, this much may be said. If it is intended to procure a Dallmeyer, it is only necessary to know the limit in size of the work to be done. If in portraiture, you select a lens that will cover the size of plate you intend to use. If in landscape, or architectural work, or copying, consult the catalogue and order the lens that meets the requirements of the case. There is said to be no difference in these lenses; that is, all lenses of the same series and size are exactly alike, the glass is of the same density, ground to the same curve, and polished to the same degree of fineness, so that each one is as good and no better than any of the others of the same kind and size. It is the same with the view-lenses; one may order with confidence, and not be disappointed by receiving an inferior lens, while expecting the best in the world.
It is not so, however, with any other make of lenses; at least I have never heard such a claim made on the part of any other maker, hence for the selection of a lens by any other maker, some thought and experiment are justifiable and even necessary. Under such circumstances, the following method of testing a lens will be found useful.
When of several lenses of the same size it is desired to select the best, attach one to a camera box and focus it on some long object placed parallel with the axis of the lens; adjust the focus so that the part of the object nearest the lens shall be in focus, but near that point where it would begin to lose sharp definition. Mark the position of the carriage on the rail, then turn the focus forward so that the same point will be as near the other extremity of sharp definition, then mark the position of the carriage on the rail and note the distance between the two marks, which will indicate the depth of sharp focus. Now stretch a line across the room and focus the instrument on the centre of the line, and note to what distance on each side of the centre sharp definition extends; this will indicate the flatness of the field. Try all the lenses in the same manner, and secure the one that has the greatest depth of focus and the greatest extent of sharp definition on the horizontal line.
Expensive lenses should be treated with much carefulness. They should be kept free from dust and dampness, and should never be cleaned or rubbed with cloth or the handkerchief; nothing but fine chamois should be used to clean a lens, the fine polish of which (one important factor in its usefulness and value) is so easily injured by abrading its surface with anything of a fibrous or gritty nature.
The lens should always be covered when the day's work is done and while the studio is being cleaned up, to exclude dust, etc., from the glasses.
Examine the lens every morning to see if the glasses are at all dimmed by damp or dust, and if so, use the chamois. You must not expect good work from lenses whose surfaces are in any way dimmed.
Study your lens with care, and learn all its good and weak points, and so enable yourself to take every advantage of instrument, light and pose in making a sitting.