The focal length of an optical system is a measure of how stron... - dofaq.co
focal length

The focal length of an optical system is a measure of how stron...

wikipedia - 07 Sep 2015
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated (parallel) rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more sharply, bringing them to a focus in a shorter distance.

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