[Illustration: MELTON MEADOWS. A. HORSLEY HINTON.]
_The Comparison and Use of Lenses._--The optician when he tests lenses looks for each fault individually, but this the ordinary photographer is hardly able to do, nor is it particularly desirable for him, because if a lens is inferior it matters little to him why it is so. On the other hand occasion may arise when he wants to identify a fault, then the information already given will probably be sufficient to enable him to do so, if to it is added that a small pinhole with a flame behind it is a convenient point of light, and that if the image of this luminous point is examined with a good eyepiece, without the focussing screen, at various parts of the field, the character of the defect may be discovered.
The main things that the photographer needs to look to in judging of a lens or comparing it with another, are (1) that it works to focus, (2) the quality of its defining power especially towards the edges of the plate. There must also be taken into account the focal length and aperture, and if both these are not the same in the lenses to be compared they should be nearly the same, and the proportion that the aperture bears to the focal length should be exactly the same. A special diaphragm may have to be cut out of card for one of them. The best test object that is always at hand is a newspaper pinned flat against a flat wall. The camera must not be moved during the work. Each lens is very carefully focussed and a negative made, using the same aperture, time of development, and in all ways similar treatment for both. If the focal lengths are different, the images will be of correspondingly different sizes, and then the same detail must be compared, not the definition at the same distance from the centre.
All good lenses work to focus, but some of the cheaper ones do not. To test this, any series of small objects arranged side by side, but at distances varying by intervals of say two inches from the camera, is photographed after carefully focussing on the middle one. If any other than the middle one is the best defined, the lens is at fault. But in this, as in all similar tests, it must be remembered that ordinary dry plates are not quite flat, and the error of the plate may make an appreciable difference.