ABSTRACT
Increasing the size of a pinhole from the optimum size allows more light to pass, which will increase the illuminance at the fi lm plane and reduce the exposure time, but it also reduces the image sharpness. Decreasing the pinhole size, however, does not increase image sharpness. When the size is decreased below the optimum size for the specifi ed pinhole-to-fi lm distance, diffraction causes a decrease in sharpness (see Figure 4-3). The optimum
pinhole size (Eq. 4-1) can be calculated with the formula
D f
(Eq. 4-1)
where D is the diameter of the pinhole in inches, and f is the pinhole-to-fi lm distance in inches (see Table 4-1).* For example, with a pinhole-to-fi lm distance of 8 inches, the diameter of the optimum size pinhole is about 1/50 inch. A No. 10 needle (a very fi ne sewing needle) will produce a pinhole of approximately this size.
