The American annual of photography . Fig. 1 AA being the lens, BB and CC being the plane of focusof the chemical and visual rays, respectively. The sim-plest way to test a lens for this defect is to it in a cameraand focus on a dark colored tree trunk with a light back-ground. Here the rays used in focusing on the ground-glassare essentially the yellow, or non-actinic, rays. Now, if we put 35 a sensitive plate in the position of the ground-glass, and makean exposure, the rays acting on the silver salts of the emulsionwill be affected chiefly by the blue and bue violet rays. Hence,when th
The American annual of photography . Fig. 1 AA being the lens, BB and CC being the plane of focusof the chemical and visual rays, respectively. The sim-plest way to test a lens for this defect is to it in a cameraand focus on a dark colored tree trunk with a light back-ground. Here the rays used in focusing on the ground-glassare essentially the yellow, or non-actinic, rays. Now, if we put 35 a sensitive plate in the position of the ground-glass, and makean exposure, the rays acting on the silver salts of the emulsionwill be affected chiefly by the blue and bue violet rays. Hence,when the exposure is made, and the lens is not corrected, thetree upon which it was focused will be blurred and ruined LANDSCAPE. JAMES T. DYE. There are two simple ways of overcoming this difficulty, thefirst being to place, while focusing, in front of the lens a pieceof blue violet glass. This permits only the chemical rays toenter, hence the view may be focused by chemical rays , however, has its disadvantage, for the amount of lightadmitted is so small that it is difficult to get a good, sharpfocus. The other method is to focus on an object, and makean exposure, then rack the plate forward about i-i6th of aninch, and make a second exposure on the second plate. Re-peat this several times, moving it one i-i6th of an inch eachtime. After development it can be readily seen which has tfiebest definition. Make a note of this, and each time in thefuture when using the lens after focusing rack the plate lor- 36 ward this constant amount. One thing that is very importantto bear in mind is when using a lens having this defect, thesmallest stop consistent should be used; as this trouble comeslargel
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