accepted circles of confusion are given in Table 3-4 below.
f. The smaller the film size the smaller must be the circles of
Since small film must be enlarged in order to produce usable
As an example,
take a 35mm negative. It has a usable area of 24mm by 36mm (about 1" by 1
7/16") (fig 3-8). When this area is enlarged or projected on a screen it
will have been "enlarged" anywhere from 4 to 40 diameters. Consider that a
4- by 6-inch print is roughly a 4 diameter enlargement of a 35mm negative.
If you projected a 35mm slide on a 40-inch by 60-inch screen, your
"enlargement" is about 40 diameters. In other words that very small circle
of confusion is now about 40 times bigger when viewed on a screen. This is
why CoC and focus are important to a photographer.