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.
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.
Source: Wikipedia "Focal length" · CC BY-SA 4.0
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