Enter the focal length (mm) and the aperture (mm^2) into the Focal Ratio Calculator. The calculator will evaluate and display the Focal Ratio.

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## Focal Ratio Formula

The following formula is used to calculate the Focal Ratio.

**FR = FL / A**

- Where FR is the Focal Ratio
- FL is the focal length (mm)
- A is the aperture (mm^2)

## How to Calculate Focal Ratio?

The following example problems outline how to calculate Focal Ratio.

Example Problem #1:

- First, determine the focal length (mm).
- The focal length (mm) is given as: 5.

- Next, determine the aperture (mm^2).
- The aperture (mm^2) is provided as: 50.

- Finally, calculate the Focal Ratio using the equation above:

FR = FL / A

The values given above are inserted into the equation below and the solution is calculated:

FR = 5 / 50 = .10

Example Problem #2:** **

For this problem, the variables required are provided below:

focal length (mm) = 6

aperture (mm^2) = 70

Test your knowledge using the equation and check your answer with the calculator above.FR = FL / A** = ?**