Calculate maximum focal length from optical zoom and minimum focal length, or solve for zoom ratio or the wide end.

Use the same focal-length basis for both endpoints: actual mm or equivalent mm. A zoom ratio alone cannot determine focal length.

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 = SFL × OZ; OZ = LFL ÷ SFL; SFL = LFL ÷ OZ

Variables:

  • 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.

Optical Zoom to Telephoto Focal Length (mm) (Assumes Wide = 28 mm)
Optical Zoom (X) Tele Focal Length (mm)
1.5×42.0
2×56.0
2.8×78.4
3×84.0
3.5×98.0
4×112.0
5×140.0
6×168.0
7×196.0
8×224.0
9×252.0
10×280.0
12×336.0
14×392.0
15×420.0
18×504.0
20×560.0
24×672.0
30×840.0
40×1120.0
*Assumes Wide (Min) focal length = 28 mm. Tele (mm) = Optical Zoom × Wide (mm). Example: 3× at 28 mm = 84 mm.

What is Optical Zoom?

Optical zoom changes a lens’s focal length. Its zoom ratio is the longest focal length divided by the shortest, so a ratio alone does not specify millimeters. Use the same actual or equivalent focal-length basis for both endpoints. Optical zoom differs from digital cropping; image quality still depends on lens performance, focus, exposure and camera settings.

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) = 5×. Maximum focal length = 35 mm × 5 = 175 mm. A zoom ratio must be at least 1; maximum focal length cannot be smaller than minimum focal length.