What is the f-number?

The f-number (also called f-stop or focal ratio) is the ratio of a lens's focal length to its aperture diameter: N = f / D. A lower f-number means a wider aperture and more light reaching the sensor or film.

Why does a lower f-number let in more light?

Light intensity at the focal plane is inversely proportional to the square of the f-number. For example, f/2 gathers four times more light than f/4, because the aperture area is four times larger.

What units should I use?

Focal length and aperture diameter must use the same unit. The f-number itself is dimensionless, so the unit cancels out. This calculator supports millimeters, centimeters, and inches.

What is a fast vs. slow lens?

A "fast" lens has a low f-number (e.g., f/1.4 or f/1.8), meaning a large aperture relative to its focal length, which allows more light and shorter exposure times. A "slow" lens has a high f-number (e.g., f/16 or f/22).

How does f-number relate to depth of field?

A lower f-number (wider aperture) produces a shallower depth of field, useful for portrait photography. A higher f-number (narrower aperture) increases depth of field, keeping more of the scene in sharp focus — common in landscape photography.

F-Number Optics Calculator

Enter any two of focal length, aperture diameter, or f-number to calculate the third value instantly.

Optical Parameters

Enter any two values — the third is calculated automatically.

Solve for

Length Unit

Focal Length

Aperture Diameter

F-Number (f-stop)

(N)

Results

Standard F-Stop Scale

F-Stop	Relative Area	Light (stops from f/1)

Summary

The f-number (focal ratio or f-stop) describes the relationship between a lens's focal length and its aperture diameter using the formula N = f / D. This calculator solves for any one of the three variables when the other two are known, making it useful for photographers designing custom lenses, telescope builders choosing mirrors, and optical engineers specifying components. Results are instant and support millimeters, centimeters, and inches.

How it works

Select which value you want to calculate: f-number, aperture diameter, or focal length.
Enter the two known values in the input fields.
The calculator applies the formula N = f / D, where N is the f-number, f is the focal length, and D is the aperture diameter.
Click Calculate (or press Enter) to solve for the missing optical value and view f-stop comparisons.
Use the unit selector to work in millimeters, centimeters, or inches.

Use cases

Determine the f-number of a custom lens given its focal length and physical aperture size.
Calculate the required aperture diameter for a telescope to achieve a specific f-ratio.
Find the focal length needed to reach a target f-stop for a camera lens design.
Compare light-gathering capability across different optical instruments.
Design microscope objectives or telescope mirrors with a given f-ratio.
Verify manufacturer specifications for camera lenses and telescopes.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu