Focal length

Measure of how strongly an optical system converges or diverges light

The focal length of an optical system is a measure of how strongly the system converges or diverges light; it has units of length, and for an idealized thin lens is equal to the distance between the lens and its focal points. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly.

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