What does it mean?

What does “EOS” mean?

Canon’s line of autofocus-capable SLR cameras is sold under the name EOS. This stands for “electro-optical system” but is also meant to be a reference to Eos, a Greek goddess of the dawn. Some people pronounce it like the goddess (ee-oss) and others as separate letters (ee-oh-ess).

Note also that the company itself is Canon with one N. In its very early days it was named Kwanon, after the Buddhist goddess of mercy. However the company soon changed to Canon (a general law or principle).

What does “EF” mean?

Lenses built by Canon for use with their EOS series of cameras are technically known as EF-series lenses. This acronym stands for “electrofocus.” Older Canon lenses which are not marked EF, such as FD and FL series lenses, are not compatible with EOS cameras.

There are five minor points of note here. Mainly of interest to completists, but there we go.

• First, Canon sell expensive specialized TS-E and MP-E lenses which, while technically not EF lenses since they lack autofocus motors, are nonetheless designed for EOS cameras.
• Second, while Canon autofocussing lenses are technically known as EF you will sometimes see them referred to as “Canon autofocus” lenses. Third-party lensmakers may or may not use the EF specification - they might simply refer to their lenses as being “For Canon EOS” or “Canon autofocus compatible.”
• Third, Canon briefly sold a camera with an EF lens mount which lacked autofocus circuitry. This camera, the EF-M, could accept all EF lenses but you had to focus them manually.
• Fourth, in 2004 Canon introduced the digital EOS 300D/Digital Rebel/Kiss Digital camera, which included a new EF lens mount variant known as EF-S. An EF-S mount camera can accept both EF and EF-S lenses, but all other EOS cameras take only EF lenses.
• Finally, just to confuse things further, in 1973 Canon released a manual-focus camera which was called the Canon EF. It predates the EOS system by 14 years and it cannot use EF lenses.

What does “SLR” mean?

All Canon EOS cameras are SLRs, which stands for “single lens reflex.” Very simply an SLR is a camera in which there is only one lens, which is used for both picture-taking and viewfinding. When you peer through the viewfinder at the back of the camera you’re looking directly through the main picture-taking lens, so you can see pretty well exactly what’s going to be on film. There isn't a separate viewfinder lens on the front of the camera like on a point and shoot camera.

The word “reflex” in there refers to a mirror used to reflect light from the lens up into the viewfinder. SLRs also have glass pentaprisms or pentamirrors on the top, which explains the protruding section on top of the camera.