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Focal Ratio Calculator

Focal ratio (f-number) of a telescope or lens from focal length and aperture, with exposure impact.

Enter values

Results update as you type.

Result

Focal ratio

f/8

As a number
8
Speed
Medium — good all-rounder
Light gathering vs f/8
1 ×

About the Focal Ratio Calculator

Focal ratio (f-number) of a telescope or lens from focal length and aperture, with exposure impact.

Also known as: f number calculator, f ratio telescope, aperture to f stop, focal ratio formula.

How to use this calculator

  1. Enter solve for.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

f-ratio = focal length ÷ aperture

Worked example

Using the values pre-filled in the calculator above:

  • Solve for: Focal ratio
  • Focal length = 1,200 mm
  • Aperture = 150 mm

the calculator returns:

  • Focal ratio: f/8
  • As a number: 8
  • Speed: Medium — good all-rounder
  • Light gathering vs f/8: 1 ×