Enter the smallest focal length and the optical zoom value into the calculator to determine the largest focal length in millimeters. This calculator helps photographers and videographers to understand the range of a zoom lens.

Optical Zoom to Focal Length Formula

The following formula is used to calculate the largest focal length based on the optical zoom and the smallest focal length of a lens.



  • LFL is the largest focal length (mm)
  • SFL is the smallest focal length (mm)
  • OZ is the optical zoom (x)

To calculate the largest focal length, multiply the smallest focal length by the optical zoom factor.

What is Optical Zoom?

Optical zoom refers to the actual increase in the focal length of a camera lens. Unlike digital zoom, which simply enlarges the pixels of an image, optical zoom adjusts the lens to bring the subject closer, without losing image quality. It is a crucial feature in photography that allows for more detailed and close-up shots without moving closer to the subject.

How to Calculate Largest Focal Length?

The following steps outline how to calculate the Largest Focal Length.

  1. First, determine the smallest focal length (SFL) of the lens in millimeters.
  2. Next, determine the optical zoom (OZ) factor of the lens.
  3. Use the formula from above = LFL = SFL * OZ.
  4. Finally, calculate the Largest Focal Length (LFL) in millimeters.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

smallest focal length (SFL) = 35 mm

optical zoom (OZ) = 5x