Cut a circular disc of white paper about three inches in diameter, and attach it to a black background. Focus this disc on the center of the ground-glass. Turn the camera until the disc is seen at the margin of the plate. If it is indistinct, and on attempting to focus it spreads out horizontally or vertically (becomes oval in shape), the lens has not been corrected for astigmatism.

Centering of Combination

To learn whether the different combinations of a lens have been properly mounted and centered, hold the lens at arm's length and about two feet from a candle flame. If the lens cells have been properly mounted, the images of the flame in the different glasses will be one behind the other. By slightly turning the lens from one side to the other you will be able to carry out this test successfully.

Chromatic Aberration

Place three white cards, about two inches apart, one behind and slightly above the other, lettering each with a black letter. Focus on the middle card, having the camera about four feet distant. Expose a plate and develop it. If the center card is not the sharpest the lens is not properly corrected for chromatic aberration.

Covering Power

To test for covering power place the lens in a large camera. Without stopping the lens down at all it should sharply and evenly cover an area on the ground-glass equal to the size for which it is listed; i. e., if the lens is listed to cover a 5 x 7 plate the corners, as well as the sides, of a space on the ground-glass of 5 x 7 inches in size should be perfectly sharp.

Curvilinear Distortion

Focus the corner of a building sharply on the center of the ground-glass (any straight upright object will answer). Turn the camera until the corner of the building is brought to the edge of the ground-glass. The lens has not been corrected for this distortion if the edge of the building is not perfectly straight.


Flare, which is a round, semi-opaque spot appearing in the center of a developed plate, may be tested for by pointing the camera to a strong light. If a circular patch of light appears on the ground-glass the lens has not been corrected for flare.

Flatness of Field

This may be tested for in the same manner as curvilinear distortion, the image remaining perfectly sharp at the edges of the ground-glass after the camera is rotated.

Focal Length

To find the focal length of a lens, carefully focus it on a distant object - one at least 150 feet away. The distance from the center of the lens combination to the ground-glass is the approximate focal length of the lens.

The Glass

To test the quality of the glass remove one of the lens combinations and place it on a sheet of pure white paper. There should be no perceptible color, as the least trace of yellow is extremely detrimental, for it reduces the speed of the lens to a considerable degree. The surfaces of the lens glasses should be free from scratches and have a high polish. Minute air bubbles occur in the highest grade of lenses, but if these bubbles are not large, nor occupy a central position in the lens, they are not detrimental.