Enter the radius of curvature (mm) into the Focal Distance Calculator. The calculator will evaluate and display the Focal Distance. 

Focal Distance Formula

The following formula is used to calculate the Focal Distance. 

FD =  R / 2

  • Where FD is the Focus Distance (mm)
  • R is the radius of curvature (mm) 

How to Calculate Focal Distance?

The following example problems outline how to calculate Focal Distance.

Example Problem #1:

  1. First, determine the radius of curvature (mm). 
    • The radius of curvature (mm) is given as: 55.
  2. Finally, calculate the Focal Distance using the equation above: 

FD =  R / 2

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

FD = 55 / 2 = 27.5 (mm)


Example Problem #2: 

For this problem, the variables needed are provided below:

radius of curvature (mm) = 89

${V2} = ${number 4}

This example problem is a test of your knowledge on the subject. Use the calculator above to check your answer. 

FD =  R / 2 = ?