d. If a lens is focused on a nearby object (short distance setting), the
depth of field is also short.
If the distance setting is increased (lens
focused on a more distant object), the depth of field increases (fig 2-25).
For this reason, it is important to focus more accurately for pictures of
nearby objects than for distant objects.
e. Accurate focus is also essential when using a large lens opening.
small f/stop compensates for minor inaccuracies.
If enlargements are to be
made from a negative, focusing must be extremely accurate.
2-18. FOCUSING LENS ON SEVERAL OBJECTS.
When you are focusing a lens on
several objects that are at different distances from the lens, you must effect
a compromise. You usually obtain the best results by focusing on a point one-
third (1/3) of the distance between the nearest (C) and farthest (A) point of
focus, (fig 2-26).