Lens Speed

The speed of a lens is determined by the maximum aperture of the lens at a given focal length.

For example a 50mm f/2.0 lens is faster than a 50mm f/2.8 lens, because the first one has a smaller f-number (larger aperture), which means that it allows more light through the camera’s diaphragm and onto the sensor or film, allowing the photographer to set a faster shutter speed.