Back Focal Length Calculator

Calculation of Back Focal Length

Calculate back focal length (BFL) by using simple optical calculator online.

The distance from the second lens to the rear focal point of the combined system is the back focal length (BFL).

If d tends to zero, the focal lengths tend to the value of f given for thin lenses in contact.

If the separation distance is equal to the sum of the focal lengths (d = f₁+f₂), the FFL and BFL are infinite.

$B F L = \frac{f_{2} (d - f_{1})}{d - (f_{1} + f_{2})}$

where,

f₁ = Focal length of one of the lenses

f₂ = Focal length of the other one of the lenses

d = Distance between the lenses

BFL = Back focal length