There is no arbitrary rule for finding the distance of the stop from the lens. It is best discovered by experiment; the point chosen is where the maximum of sharpness is given with a minimum of distortion. If distortion is of little consequence, the stop may be brought forward until its circle of illumination just covers the plate and no more. The experiment may be made in the following manner: - Mount the lens square in a tube and then choose another tube, 2 in. long, sliding into the first easily. (The second or inner tube may be made by rolling and pasting paper round a rod built up to the right size with paper.) At the end of the inner tube, which must be cut straight and true, fix a black card having cut in it an opening about one-third the diameter of the lens or about one-sixteenth the focus. This hole represents the stop, and by sliding one tube within the other the distance between the stop and the lens may be adjusted. Place the camera, parallel with a number of straight, clear lines drawn on paper about 6 in. apart and focus them without the inner tube till they are about 1 in. apart. None of the lines will be really sharp.

Insert the inner tube and push the stop close against the lens and the definition in the centre will at once be improved, but the definition at the margins will be as bad as ever. Wow slowly withdraw the stop and the definition will be seen to spread towards the margins of the screen. As this is done, however, another evil is introduced; the lines at the margins of the paper are bent inwards at the ends and outwards in the centre. This bending of the lines is known as distortion, and is the result of using a stop.