What does the F ratio of a camera mean?

Question

What does the F ratio of a camera mean? I've seen stuff like F/4 or F/8 or F/12. What does that mean?

Answer 1

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture) of an optical system expresses the diameter of the entrance pupil in terms of the effective focal length of the lens. It is the quantitative measure of lens speed, an important concept in photography.

Answer 2

It is equal to the focal length of the lens divided by its clear aperture. So a 50mm f/2 lens would have a clear aperture of 25mm. The lower the f number, the "faster" the lens is, meaning you can take a picture with a faster shutter speed or in less light. A lower f number will also give less depth of field, meaning the background will be more out of focus. Camera lenses have a diaphragm that can restrict the aperture, resulting in a higher f number.

On manually adjustable lenses there are click stops to adjust the aperture. A "full stop" consists of a change with a ratio of approximately the square root of two (~1.4), because exposure is proportional to aperture squared. So f/2.8 requires twice as along an exposure as f/2. The normal click-stop sequence runs 1.4 - 2.0 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22. Not all lenses cover this range, especially at the low end; some stop down even further.