accepted circles of confusion are given in Table 3-4 below.
Table 3-4.
f. The smaller the film size the smaller must be the circles of
confusion.
Since small film must be enlarged in order to produce usable
print, that small circle of confusion will also enlarge.
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.
Figure 3-8.
35mm Frame
48
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