F-Number Calculator
f-number N = focal length ÷ aperture diameter.
Relative exposure only.
A QUICK WALKTHROUGH
How to use this tool
- f-number N = focal length ÷ aperture diameter.
- Choose one unknown and enter the other two positive values.
- Review f-number and relative exposure ratio.
Formula
N = f / D; doubling the f-number reduces geometric aperture area by four, before transmission losses.
Valid domain
Focal length, aperture diameter, and f-number must be finite and greater than 0. Relative exposure comparison assumes the same transmission and shutter/ISO; it is not a camera exposure meter.
GOOD TO KNOW
Common questions
Does this measure exposure?
Relative exposure comparison assumes the same transmission and shutter/ISO; it is not a camera exposure meter.
What is f/2?
An f-number of 2 means the aperture diameter is half the focal length.