320px-Aperture_diagram.svgFrom Wikipedia https://en.wikipedia.org/wiki/F-number

Ever wondered where we get the f-numbers from and why they are so difficult to remember sequentially?

An ‘f’-number is a fraction  - a ratio between the size of the hole in the diaphragm of the lens and the focal length of that lens. Divide the focal length of the lens by the lens diameter to get the ‘f’ Number.

As the diaphragm hole is 'stopped-down' (gets smaller) it effectively reduces the amount of light entering the lens barrel. Since this is the mathematical ‘area’ of the hole-diameter, the numbers aren't simply half of the previous (as say for shutter speed.) By ‘stopping-down’ one stop (by a half) the light is ‘stopped’ or reduced from entering the barrel.

The typical f-Number sequence for an SLR camera lens in Full Stops is as follows:-

f1, 1.4, 2,  2.8, 4, 5.6, 8, 11, 16, 22, 32, 

If like me, you find it hard to think in numbers, then here is a tip to remember the Full-f-Stops sequence.

Each alternate number is TWO stops more than the other.

So look at them sequentially this way f-,

32

       22

16

       11

8

       5.6

4

       2.8

2

       1.4

1

Each fraction is HALF or double the other alternating between the two columns in the sequence.

I know 1/3rd stops makes it a little more complicated but at least now you can now see a pattern.

Happy Shooting

Graham Riddell

Further Reading

https://en.wikipedia.org/wiki/Diaphragm_(optics)

https://en.wikipedia.org/wiki/F-number