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

Name: F-Number Optics Calculator
Availability: InStock
Author: Nham Vu

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

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

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.
Results update instantly as you type.
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: 2026-07-23 · Reviewed by Nham Vu