F-Number Calculator (F-Stop)
Calculate f-number (f-stop) from focal length and aperture diameter, or find aperture diameter from focal length and f-number. Uses the formula N = f / D. Free online f number calculator for photography and optics.
Use F-Number Calculator ↓F-Number Formula
N = f / D
N = f-number (f-stop), f = focal length, D = aperture (entrance pupil) diameter. Use same units (e.g. mm).
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What is F-Number?
The f-number (or f-stop) is the ratio of focal length to aperture diameter: N = f / D. Smaller f-numbers (e.g. f/2) mean a larger aperture and more light; larger f-numbers (e.g. f/16) mean a smaller aperture and less light.
To get the focal length of a lens from the lens maker formula, use our focal length calculator.