Focal length of a lens  

	The focal length of the lens is usually engraved around the barrel of the lens- in this case the lens is what is termed a fixed focal length lens. Because these lenses are specifically designed to focus at one focal length they are often sharper than an equipment focal length on a zoom lens which is compromised to cope with a range of situations
	In the case of a zoom lens there are two focal lengths 19mm - 35mm, marked on the lens, which means the focal length of the lens is not fixed and can be moved from one setting through to the next.

We also see this stamped on compact digital cameras - in this case 6.1mm - 30.5mm   remember the relevance of this mm size relates to the physical size of either the film format or the CCD and is often better understood as angle of view.

Depending upon the camera format, the standard focal length of the lens varies - ( by standard we mean that it matches human perspective) - So in terms of angle of view or perspective, a 50mm Lens on a 35mm film or full frame digital camera is equivalent to an 80mm lens on a 6x6 camera

Focal Length Equivalence Table