TESTING LENSES.—The lens being the most impor tant part of the photographer's outfit it is advisable that this should be perfectly free from defects of all kinds. Lenses by well-known makers are invariably thoroughly tested before being sent out. With other kinds, however, some rough and ready means of testing it is useful. The lens should, if pos sible, be had on approval for a few days, and tested as follows : Spherical Aberration.--Fix a newspaper or something similar in front of the camera, pinning it quite flat all over. Then sharply focus the letters in the center, and then ex amine those at the sides. If blurred and indistinct, spherical aberration is present.
Chromatic Aberration.—Arrange three cards with let fers on one behind and above the other, and focus for the center. If on exposing the plate and developing, the center one does not come out the sharpest the lens is not properly corrected for chromatic aberration.
the camera to a strong light, and if a flare spot be present it will make its appearance on the focusing screen as a circular patch of light.
Curvilinear the center of the screen focus any straight upright object, and then rotate the camera till its image is brought to the edges, when it should still be perfectly straight and upright.
Flatness of field can be detected by the same method by seeing if the image also remains perfectly sharp after the camera is rotated.
Centering of discover whether the combinations in a doublet lens are correctly mounted with regard to each other hold the lens at arm's length, and some distance from a single as or candle flame. The reflected images of the flame should be one behind the other in a straight line. It is not, of course, possible to view them all when superimposed, but by a little manoeuvring it can be seen whether they gradually come together as they should.
Focal for a distant object, and then measure the distance from the diaphragm to the ground glass. This gives the focal length of the lens, and should not be less than the diagonal measurement of the largest size plate it is intended to cover.
Covering the lens in the camera and see that with the largest diaphragm it covers the plate well, even to the sides and corners.
good lens should work with as large an aperture as possible. A "rapid rec tilinear should work at f/8, and a " landscape " at The are usually made of flint and crown glass. They should not have any perceptible color. This may be seen by laying the combinations on a piece of white paper. Bubbles and striae can be detected by looking through the lens. The surfaces should be well polished and free from scratches.
The interior of the mount and the stops should be of a dead black without any reflecting parts.