| AF-S | Auto Focus Stop feature. Unique to Canon's four Image Stabilized super-telephoto lenses. Four buttons appear on the outer barrel near the front of these lenses; pushing any one will temporarily lock AF if the camera is in the AI Servo AF mode. Custom Functions on many newer EOS bodies allow these buttons to assume a variety of additional functions. |
| CaF2 | A synthetic calcium flourite crystal. CaF2 elements provide an excellent correction potential and are about two times more effective than standard UD glass (Ultra-low Dispersion) but it's also far more expensive and only used in few lenses. |
| FP | Focus Preset. A feature on the Image Stabilized super-telephoto EF lenses. The photographer can focus upon a subject and memorize that focus setting, and later return instantly to it with a brief turn of the metal "playback" ring on the lens' barrel. |
| FTM | Full Time Manual Focus. This allows the lens to be manually focused without leaving automatic focus mode and without damaging the lens. This works best with cameras that can shift the autofocus from the shutter release to another button. |
| I/R | Inner/Rear focusing ring. Focusing is performed by moving one or more lens groups positioned between the front lens group and the diaphragm. |
| IS | Image Stabilizer. Allows the lens to sense movement from "shake" or vibrations, and instantly apply an optical correction by moving a group of lens elements. |
| UD | Ultralow Dispersion glass. A digit after the 'UD' abbreviation means a number of UD glasses in the lens. |
| USM | UltraSonic Motor |
| L | These lenses use special optical technologies [such as] Ultra-low Dispersion UD glass, Super Low Dispersion glass, Fluorite elements, and Aspherical elements |
| EF | Electro-Focus. EF-series lenses for Canon EOS series. Bayonet mount with three outer lugs. |