What is f number? F/#,f stop, or speed, are all optical terms you will probably come across whether you are into photography, optics, or not at all.
Here are the important things you need to know about f number:
- It determines how much light a lens collects
- It defines the cone angle of of light focused by a lens
- It determines the Depth-of-field of a lens
F number is also proportional to another optical term called Numerical Aperture or NA.
Formerly, f number is defined as the ratio of a lenses focal length to aperture diameter. This value is relative to image plane, thus f number defines the image space f number. Similarly, the object plane f number can be calculated by multiplying the image space f number by the lenses maginifaction.
See the following links for further reading:
More about light collection, NA and their relation to f-number: