A prime lens has a fixed focal length (FFL), as opposed to a zoom lens, which has variable focal lengths. Prime lenses are generally simpler in terms of construction and have fewer moving parts, making them smaller and lighter than zoom lenses. The simpler construction also allows them to have a large maximum aperture, making them faster than zoom lenses. Most importantly, prime lenses are optimized for their one focal length, thus giving superior quality images with fewer problems with regard to aberrations (any disturbance of rays of light that result in imperfections in the recorded image).
Fast Lens
The speed of a lens and how fast it is refers to the maximum aperture of the lens. The larger the maximum aperture, the more light is delivered to the focal plane, the faster the shutter speed can be set, and the faster the lens.
Fast lenses can be advantageous over slower ones in certain shooting conditions. They excel at night and in low light conditions, as they allow more light in to expose the film, increasing the chance of a faster shutter speed and avoiding the use of a tripod.
Fast lenses were common until the late 1960s, as color film speeds available at that time were still relatively slow.