What is an F/Stop?

Ever wonder what an f/stop is, well http://en.wikipedia.org/wiki/F-number has a great explanation. This is essentially a way to express how wide open a lense is in a camera. It is a pretty simple idea. An F-number or F-stop or aperture setting of F/1 means that for a 77mm lense (as on my big Canon lenses), the thing is all the way open and lets in lights across the entire 77mm width. Similarly F/2 means that the lense is half open so lets in 77/2.

Now in a camera, every decrease of aperture by the square root of 2 (1.414) halves the amount of light the camera sees. Hence the strange sequence you see in cameras that are really just powers of 1.414. we sequences that show a doubling of light that looks like:

code. f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/22

Put another way, if you just double the size of the aperture, you get 4x more light, this is because the area is pi*radius squared, so this is a quadrupling of light each time:

code. f/1, f/2, f/4, f/8, f/16

Feel smart now?

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