Enter the radius of curvature of the surface of a mirror into the calculator to determine the focal width.

Focal Width Formula

The following formula is used to calculate a focal width: 

f = - R / 2
  • Where f is the focal width
  • R is the radius of the curvature of the surface of a mirror

To calculate the focal width, divide the radius of the curvature of the mirror’s surface by 2.

Focal Width Definition

What is a focal width?

A focal width, also commonly used to be the same as focal length (or focal distance), is a term that describes a measure of how strong a lens or mirror converges or diverges light when it hits the curvature of the lens.

A positive focal width indicates that the lens will converge light, and a negative focal width will indicate that the lens will diverge light. The positive and negative come from which side of the lens the focal point is on.

The focal point will be to the left of the lens for diverging light and to the right for converging light.

Example Problem

How to calculate focal width?

First, determine whether the focal point lies on the left or ride side of the lens for a typical diagram. In this case, the focal point is to the right, and so the lens is converging light.

Next, measure the radius of curvature of the lens. In this example, the radius is measured to be 5mm.

Finally, calculate the focal width using the formula above:

f = – R / 2

f = – 5mm / 2

f = – 2.5mm