| 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. |
| DO | Diffractive Optics - Canon uses two single-layer diffractive optical elements whose diffraction gratings are bonded together face-to-face. The diffraction that occurs with Multi-Layer Diffractive optical elements actually corrects the optical system’s chromatic aberrations and improve the image formation performance. |
| 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. |
| 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. |
| USM | UltraSonic Motor |
| EF | Electro-Focus. EF-series lenses for Canon EOS series. Bayonet mount with three outer lugs. |