F-Stop Calculator

A stop is a doubling or halving of light. Opening up one stop doubles the light and closing down one stop halves it. The same idea of stops applies to aperture, shutter speed and ISO alike.

Light scales with aperture area, which goes as one over the f-number squared. So the stop difference is two times the base-2 logarithm of the ratio of the two f-numbers. Going from f/2 to f/4 is 2 times log base 2 of 2, which is 2 stops.

The magnitude, the light ratio and the brighter aperture do not change if you swap the two. Only the sign of the stop difference flips, showing the direction of the change between the first and the second f-number.

Look at the Brighter aperture card. It always names the smaller f-number, because a smaller f-number is a wider opening that gathers more light. If both f-numbers match, it reads Equal.

The light ratio is how many times more light the brighter aperture gathers, and it is never below 1. One stop is 2x, two stops is 4x, three stops is 8x, and f/2 versus f/8 is 4 stops, or a 16x ratio.

The f-number is the focal length divided by the aperture diameter, so a bigger number is a smaller opening. That is why f/16 is much darker than f/2 even though the number looks larger.

Each button loads a standard full-stop f-number, from f/1.4 up to f/22, into the second aperture field. They are a shortcut for comparing your first aperture against a familiar full stop without typing.

Yes. The full-stop series runs f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 and on, with each step multiplying the f-number by the square root of two and halving the light.

Yes. Type any positive f-number in either field. Comparing f/2.8 with f/3.2, a common third-stop step, returns 0.3853 stops and a 1.306x light ratio.