Most of us believe that f stop and focal length are the only two parameters for judging a lens. For the uninitiated, lens speed is another deciding factor when it comes to lenses. A lens can be termed a fast lens or a slow lens.

Fast lenses are those lenses which have a higher aperture diameter opening, or a lower f stop. These lenses are termed as fast lenses as they have a higher lens opening, which allows more light intensity to enter the focal plane; thereby making the shutter speed faster. Slow lenses, on the other hand, are the ones which have a lower f stop and allow less light intensity to enter the focal plane; hence a lower shutter speed is required to get the same exposure.

A lens with f 2 will be faster than a lens with f5.6, even though both lenses are fast lenses. Prime lenses are generally faster than zoom lenses with the same f number, because zoom lenses, such as the Pentax smcl-FA 28-105mm 3.2-4.5, are compound lenses. The light in a zoom lens must pass through many lenses before it reaches the film plane, losing a lot of light in the process, so less intensity reaches the plane and shutter speed needs to be slower. Prime lenses, on the other hand, are single lenses and there is not much intensity loss; therefore, the shutter speed is faster. Fast lenses are generally more expensive than slow lenses as they have a large aperture diameter and are quite delicate; hence requiring more care and precision while manufacturing.