Enter the refractive index and the diameter of the ball lens into the calculator to determine the ball lens focal length.

Ball Lens Focal Length Formula

The following equation is used to calculate the Ball Lens Focal Length.

BLFL = n*D/(4*(n-1))
  • Where BLFL is the ball lens focal length
  • n is the refractive index
  • D is the diameter of the ball lens

What is a Ball Lens Focal Length?


A ball lens focal length is defined as the effective focal length from the focal point to the center of a ball lens.

How to Calculate Ball Lens Focal Length?

Example Problem:

The following example outlines the steps and information needed to calculate Ball Lens Focal Length.

First, determine the refractive index. In this example, the refractive index is calculated to be 1.47.

Next, determine the ball lens diameter. For this problem, the diameter is 5mm.

Finally, calculate the ball lens focal length using the formula above:

BLFL = n*D/(4*(n-1))

BLFL = 1.47*5/(4*(1.47-1))

BLFL = 3.909 mm