What is an "f-stop"?

What is an "f-stop"?

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"F-stop" means fraction and the f-stop number is the fraction of the actual opening in the lens compared to the focal length of the lens. So, let's say we have a lens with a focal length of 5 mm and we have an f-stop on the lens of 2. 5 divided by 2 is 2.5. The actual diameter where the light comes through the lens is 25 mm across. If we have an f-stop of 1, for example, on that 5 mm lens, 5 divided by 1 means there is a 5 mm hole that the light comes through. That's what the f-stop number actually means.