The red line around the barrel of a Canon L-series lens depicts more than just a high level of professional image quality. It also indicates a ground-breaking design and manufacturing input, particularly where the glass is concerned.
Fluorite has some terrific properties for lenses, notably the complete removal of residual chromatic aberration. It’s widely used in current Canon L super-telephoto lenses. However, natural fluorite contains too many impurities to be used for lens manufacturing, as large crystals can't be obtained. In 1969, Canon developed the technology to artificially 'grow' pure crystals.
Optical performance is only half the story. A lens destined for professional use must be able to withstand the rigours of a professional photographer’s working life. This means a rugged build quality that will endure the knocks of everyday use, and weather sealing to protect against every type of adverse conditions.
Professional photographers are often forced to shoot in less than ideal lighting conditions. Sometimes this can mean in low light or even at night. The L-series offers wider maximum apertures than other lenses. For example, look at the 24mm, 50mm and 85mm prime lenses. The L-series lenses all have wider apertures, allowing better low light performance.