Focal length calculator

This calculator helps you determine the focal length of a lens based on its principal parameters

Input Parameters

Calculation Results

Calculation Formula

Focal Length (f) = Objective Diameter × F-number

Where:
Objective Diameter - The diameter of the objective lens
F-number - The f-number of the lens

Focal length calculator Calculator Usage Guide

Learn how to use the Focal length calculator calculator and its working principles

How to use the calculator

  1. Enter the Objective Diameter of your lens in millimeters (mm).
  2. Enter the F-number of your lens (e.g., f/2.8, f/4).
  3. Click the Calculate button to compute the focal length.
  4. The result will be displayed in millimeters (mm).

Understanding the formula

The focal length of a lens is calculated by multiplying the objective diameter by the f-number:

f = D × f/

Where:

  • f is the focal length in millimeters
  • D is the objective diameter in millimeters
  • f/ is the f-number (f-stop)

Example

If you have a lens with an objective diameter of 50 mm and an f-number of f/2.8:

Focal Length = 50 mm × 2.8 = 140 mm

Applications

This calculator is useful for:

  • Photography enthusiasts calculating the focal length of their lenses
  • Optical engineers designing or selecting lenses
  • Researchers needing to determine the effective focal length of optical systems