What is f number

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:


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