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 (that is, a smaller minimum f-number) is called a "fast lens" because it delivers more light intensity (illuminance) to the focal plane, achieving the same exposure with a faster shutter speed. It also needs to reduce interference from extraneous (light) noise i.e. lower the noise floor. A smaller maximum aperture (larger minimum f-number) is "slow" because it delivers less light intensity i.e. has a higher noise floor and requires a slower shutter speed.