Concept of Aperture Value

What is the relation between the aperture value (f/3.5, f/8 and so on) and the geometrical size of the aperture? Does this change from lens to lens?
K T Thomas, Kerala

The diameter of the aperture opening differs in lenses of different focal lengths. There is a simple formula that helps you understand the size of an aperture opening at a given aperture setting. If you use a 50 mm lens at f/4, the diameter opening of the diaphragm will be 50/4 = 12.5mm. To give another example, a 200mm lens at f/4 will have a diaphragm diameter opening of 200/4, that is, 50mm.

This is actually the reason why so many beginners get confused about the relation between f/number and aperture size. A smaller f/number like f/4 actually means a wider opening than an aperture value like f/22. This is why more light enters the lens while using a wider aperture like f/4, as compared to the use of a narrow aperture of f/22.

This article originally appeared in the March 2015 issue of Better Photography.