Remember the main aperture numbers? f1.4, f2.0, f2.8…
No, let’s do that again properly:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, and so on.
(Note: Corrected the f/35, my typo)
The last list is correct since an aperture number is a fraction. Strictly speaking, it is not “f 8”, but “f divided by 8”. Where, simply explained, f is the focal length of the lens and the result of the fraction is the diameter of the lens opening.
Why are these numbers important?
Because every next number is a stop darker (or when you move to the left in the list, a stop brighter) than the previous number.
A stop means halving or doubling the light. Hence the funny numbers; the ratio between these numbers is √2, or around 1.4, since to halve a circle’s area you reduce its radius by √2; halving the radius would instead you a quarter of the area (πr2).
So why are these numbers good to know? Well, imagine you are shooting at f/5.6 and 1/50th of a second. If instead I wanted to go to 1/100th second, what f-number would give me the same exposure?
The answer: 1/50th to 1/100th sec is one stop less light. So the aperture needs to provide one stop more light. Which means f/4, and you can set that instantly if you know the table of main values. Hence its importance!
Happy July 4th, US friends!