Enter a base (reference) focal length and either a resulting focal length or a focal-length multiplier (“zoom factor”) to solve for the missing value. This calculator compares one focal length to another using a simple focal-length ratio (a dimensionless multiplier). Note: this is not the same as a zoom lens’s “zoom ratio” (max focal length ÷ min focal length).
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Zoom Factor Formula
The following formula is used to calculate the zoom factor (a focal-length multiplier relative to a chosen reference focal length).
ZF = R / B
Variables:
- ZF is the zoom factor (×), a dimensionless multiplier
- R is the resulting focal length (mm or in)
- B is the base (reference) focal length (mm or in)
To calculate the zoom factor, divide the resulting focal length by the base (reference) focal length. Use the same unit for both focal lengths so the units cancel.
| Base Focal Length (mm) | Zoom Factor (×) | Resulting Focal Length (mm) |
|---|---|---|
| 12 | 2.0 | 24.0 |
| 16 | 1.5 | 24.0 |
| 18 | 1.4 | 25.2 |
| 24 | 1.7 | 40.8 |
| 28 | 2.0 | 56.0 |
| 35 | 1.4 | 49.0 |
| 40 | 1.5 | 60.0 |
| 50 | 2.0 | 100.0 |
| 55 | 1.7 | 93.5 |
| 70 | 1.4 | 98.0 |
| 85 | 2.0 | 170.0 |
| 100 | 1.5 | 150.0 |
| 105 | 1.4 | 147.0 |
| 135 | 1.7 | 229.5 |
| 150 | 2.0 | 300.0 |
| 200 | 1.4 | 280.0 |
| 250 | 1.5 | 375.0 |
| 300 | 2.0 | 600.0 |
| 400 | 1.4 | 560.0 |
| 600 | 2.0 | 1200.0 |
| Formula: R = B × ZF, where R is resulting focal length, B is base focal length, and ZF is the zoom factor. Values rounded to one decimal place where applicable. | ||
What is Zoom Factor?
On this page, “zoom factor” means a focal-length multiplier: the ratio of one focal length to a chosen reference (base) focal length. Because angle of view (and image size on the sensor for distant subjects) scales approximately with focal length, this ratio can be used to describe how much narrower the field of view becomes compared to the reference focal length on the same camera. It is dimensionless (for example, 2× or 4×). Note: this is different from a zoom lens’s zoom ratio, which is the lens’s maximum focal length divided by its minimum focal length.
How to Calculate Zoom Factor?
The following steps outline how to calculate the Zoom Factor.
- First, determine the resulting focal length you want to compare (R) in millimeters (or inches).
- Next, choose the base (reference) focal length (B) in the same unit. For context, a “normal” lens is often around 50 mm on full-frame, around 35 mm on many APS‑C cameras, and around 25 mm on Micro Four Thirds.
- Next, gather the formula from above: ZF = R / B.
- Finally, calculate the Zoom Factor (ZF).
- 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.
focal length of the lens (R) = 200mm
base (reference) focal length (B) = 50mm, so ZF = 200 / 50 = 4×
