ABSTRACT
Calculating F-Numbers F-numbers are calculated by dividing the lens focal length (f) by the effective aperture (D), or f-number f/D. Since the effective aperture is rarely the same size as the aperture (the physical opening in the diaphragm), the diameter of the entering beam of light that just fi lls the diaphragm opening must be measured. One method to determine this diameter is to place a point source of light one focal length behind the lens so the diaphragm opening restricts the beam of light emerging from the front of the lens. This beam will be the same size as an entering beam that just fi lls the diaphragm opening (see Figure 2-3). The diameter of a beam of light can be measured with a ruler in front of the lens.
