LENSSPEED
Lens speed
Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture is called a "fast lens" because it delivers more light intensity to the focal plane, achieving the same exposure with a faster shutter speed. A smaller maximum aperture is "slow" because it delivers less light intensity and requires a slower shutter speed.The above text is a snippet from Wikipedia: Lens speed
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