Editorial Feature

What is a Focal Length?

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Article updated on 03/03/20 by Jo Finchen-Parsons

The focal length of an optical system is a measure of how strongly it focuses or diffuses light. A system has greater optical power if it has a shorter focal length than a system that possesses a longer focal length. The focal length can also be used to calculate the magnification of a lens system.

The mathematical approach to calculating the focal length is relatively simple. The focal length can be expressed using the following relation:

(1/image distance) + (1/object distance) = (1/focal length)

The focal length of a lens system can also be used to determine the system’s power. The power of a lens system is calculated by dividing the refractive index in the image space by the focal length. The refractive index is a measure of the refraction of a ray of light as it passes from one medium to another.

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