High Quality Content by WIKIPEDIA articles! Lens speed refers to the maximum aperture diameter, or minimum f-number, of a photographic lens. A lens with a larger maximum aperture (that is, a smaller minimum f-number) is a fast lens because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity and requires a slower shutter speed.