Since your previous camera was a 35mm film camera, it used lenses which produce a fairly large "image circle" inside the camera body. The image had to be large enough to completely cover the frame of 35mm film.
On a digital camera, a "full frame" sensor body (such as a 6D, 5D, or several of the 1D bodies) have sensors which are the same size of a frame of 35mm negative film.
The lenses that project this large image circle suitable for use on any 35mm EOS "film" camera or full frame EOS "digital" camera are called "EF" series lenses (they are still "EOS" lenses... but they have a model prefix... such as "EF 50mm f/1.4 USM" -- notice the "EF" at the front of the name.
Canon EOS "EF" series lenses can be used on ANY Canon "EOS" body -- regardless of whether it is a film body or a digital body and all functions will work. e.g. if it's an auto-focusing lens then the auto-focus will work. The camera will be able to control aperture settings, etc. You can think of these "EF" lenses as being universal to all EOS bodies.
But there is another sensor size which is extremely common.
All "Rebel" series cameras as well as all mid-level cameras (the 20D, 30D, 40D, 50D, 60D, and 70D) as well as the 7D, all use "APS-C" size sensors. This sensor is roughly the size of a single frame of "Advanced Photo System - Classic" size film, which is a bit smaller than 35mm film. Since the sensor is smaller, they have the ability to use a lens which projects a smaller image circle.
These cameras can use lenses which project a smaller image circle because they have a smaller sensor size. Canon makes a series of lenses intended for these smaller sensors. These are "EF-S" series lenses. You'll know them becauase the name has an "EF-S" prefix such as "EF-S 18-55mm f/3.5-5.6 STM" (notice the "EF-S" prefix in front of the name). These lenses are able to take advantage of cost savings because the lenses do not need to project a large image circle.
While all "Rebel" bodies and all mid-level bodies (except the 10D) can use "EF-S" lenses, they can ALSO use "EF" lenses (remember... "EF" lenses are universal to all Canon EOS bodies). The 10D is the only exception but that's a very old camera and hasn't been in production for years.
Tim Campbell
5D III, 5D IV, 60Da
