Short tips for F-number, Diameter of aperture and the amount of light

Source: http://en.wikipedia.org/wiki/F-number#Stops.2C_f-stop_conventions.2C_and_exposure

Here is a short diagram for showing the relationship between F-number and what's happening in the real world...

F-number = f/D
where f is the focal length, and D is the diameter of the entrance pupil (aperture)

let's take a simple example:
if
F-number = 1, f = 100mm, D = 100mm, Aperture = Pi*(D/2)^2 = 2500Pi
So in order to get an aperture with half size,
1250Pi = Pi * (D'/2)^2, and
2 * Pi * (D'/2)^2 = Pi * (D/2)^2
D' = 1/(2)^0.5 * D

This gives about D' = (1/1.4) * D

and this is so called "1-stop" decrease in light! Thanks for reading!


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