Question: what is the formula for determining a lens’ f-number of f-stop?
Answer: F − number = f D 1 Page 2 APERTURE AND F-NUMBER 2 If the area of an aperture is doubled, the number of photons that can reach the sensor through the lens is also doubled. The F-number of a lens is the ratio of its focal length divided by the diameter of the aperture (Figure 1).