Canon sell a number of lenses in a special series they refer to as L for “luxury.” These are their most expensive and highest-quality lenses, and are readily identifiable by the red stripe painted around the end of the barrel.

L series lenses offer higher optical quality than their non-L equivalents, and have an important technical aspect in common. At least one element in every L lens is either made of fluorite crystal rather than glass, is a ground aspheric lens element (not a moulded/replicated aspheric lens as used in less expensive lenses) or is made from ultra-low dispersion glass. Most L series lenses are also sturdily built - many are encased in metal barrels and are weatherproofed - and most are very fast lenses for their focal lengths. Nearly all telephoto L series lenses are also off-white rather than black.

These lenses are, therefore, marketed as professional camera lenses and are usually priced out of the range of most consumers. They can be used to take great photographs, but the cost, weight and size of these lenses are the tradeoffs.

Of course, a lens doesn't have to be an L series lens to take good pictures. Many EOS lenses offer excellent optical quality - they just don't need and thus don't have exotic fluorite lens elements and so on. Many of Canon's prime lenses in the 35mm to 135mm range fit in this category - see below. And some recent EF-S lenses offer near-L image quality but lack the red ring and the tough build quality of contemporary L series lenses.

Note also that the presence of a red ring around the end of a lens barrel only indicates an L series lens if it's actually made by Canon. Some other makers happily paint red stripes around the end of their lenses too, but this in no way guarantees that the lens meets the quality standard of a Canon L lens.