The correct distance at which a stop should be placed from a lens is that which would give the maximum of covering power with a minimum of distortion. If a cardboard stop is placed close against the lens and moved gradually from it the best position will readily be found, for it will be seen that as the stop recedes from the lens the sharpness spreads to the edges, but straight lines coming near the margins are bent outwrards in the centre. The accompanying- diagram shows another method of working out the correct position of the stop. Construct a square AGCF, the sides of which are equal to the focus of the lens. Draw the diagonal B and a line H equal to the focus. At the end of this line draw DE. equal in length to the diameter of the lens. If lines now drawn from F and G through E and D, the point where they meet X is the position for the stop.

Distance of Stop from Lens.