Themost primitive way of mounting a gelatin filter is to cut a disc from a sheet of dyed gelatin and insert it between the components of the lens. For this purpose the gelatin must be perfectly flat, which is insured by its method of preparation and test. One disadvantage of this method is that the filter can be inserted and removed only upon the ground. It is less satisfactory the larger the diameter of the lens, and the wastage of filters due to insertion and removal is apt to be high. The camera should be refocussed after filters of this kind are inserted.
Glass filters, ground optically true, or gelatin filters, mounted between optically flat glass plates, are the most convenient and satisfactory. They may be mounted in circular cells to screw or attach by bayonet catches to the front of the lens. Or they may be mounted in rectangular frames to slide into transverse grooves in the camera body. Fig. 44 shows the mount of this latter form adopted in the larger United States Air Service cameras. This is particularly convenient if it is desired to insert or change the filter while in the air—a practice not generally considered feasible in war work with the photographically inexperienced observer, but likely to be common with the employment of skilled photographers for peace-time aerial photography.
German cameras are reported in which the glass filter is carried behind the lens, on a lever which also carries a clear glass plate of the same thickness, to be thrown in when no filter is needed, thus maintaining the focus. The performance of the lens will be impaired by this scheme, unless it is specially calculated to offset the effect of the glass introduced in the path of the rays behind the lens—optically true glass has no effect on definition if placed in front of the lens. Glass filters may also be placed in close contact with the plate or film, in which case they must be much larger, but do not need to be of as good optical quality.