Have you ever wondered why the lens openings on a camera are called f-stops? If so, read on. I learned that in 1858 John Waterhouse invented a system of metal discs with different sized holes to act as the aperture when dropped into a slot in a lens. He called them “Waterhouse Stops” because the discs were literally stopping light from entering the camera. This is likely the first instance of the use of the word ‘stop’ as it relates to aperture and it seems to have stuck.

In 1895, John A. Hodges proposed the ‘fractional number’ system (which he abbreviated to ‘F-number’). This is the first recorded instance of the ‘fractional number’ and might be the original meaning of the ‘f’ in F-stop, since aperture openings are measured as fractions of the focal length of a lens. Thus the ‘f’ stands for the, ‘focal length’ in the aperture rating. A stop in photography nomenclature means a difference of one exposure value, which is the doubling, or halving, of the amount of light reaching the sensor. A setting f-2 means the diameter of the focal length is divided by two. Here are a few standard “full stops” that f-numbers are rated in:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64

These aperture settings all differ by one full exposure value, or one full “stop”, and create the full f-stop scale. When you close down a 50mm f/1.4 lens from its maximum aperture of f/1.4 to an aperture of f/2.8, you are allowing only half as much light to hit the sensor. It should be noted that many cameras these days offer a two additional f-stop scales beyond the standard full stop scale. Those are a half-stop scale and a third-stop scale.

Half-stop Aperture Value Scale

1, 1.2, 1.4, 1.7, 2, 2.4, 2.8, 3.3, 4, 4.8, 5.6, 6.7, 8, 9.5, 11, 13, 16, 19, 22

Third-stop Aperture Value Scale

1, 1.1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.5, 2.8, 3.2, 3.5, 4, 4.5, 5.0, 5.6, 6.3, 7.1, 8, 9, 10, 11, 13, 14,16, 18, 20, 22

Now that you know how f-stops relate to aperture and the amount of light reaching the sensor, you can use the term “stops” as a general term to refer to a doubling (selecting toward the right) or halving of light (selecting toward the left) for any setting. Well, I thought it was interesting anyway.