A buyer's guide to the Canon EOS lens system

History
The Canon EOS (Electro Optical System) was introduced in the mid-1980's
and was the autofocus system that they rolled out across the range of
35mm cameras.
Many manufacturers at the time upgraded their current fitment to
include auto focus, but Canon used the change to introduce several new
features.
They obsoleted the FD fitment, and introduced the EF fitment. For a
select few, Canon did offer a converter, really only to those
professionals that had invested in several of the high-end L grade
lenses.

Later Canon used the same fitment on their APS, and then digital cameras.

The lens is the start of the image making process. A bigger or smaller
sensor, more pixels, lower noise all make a difference, but if you
start with a poor quality lens on the front, you'll be making life hard
for yourself.


Compatibility

• A Canon EF lens will work on any Canon EF or EF-S body.
  
• A Canon EF-S lens will work on any Canon EF-S body (and won't fit an EF body).
  
• Non-Canon EF lenses will normally work on bodies produced upto
  when the lens was released. Sometimes it will work on more recent
  cameras.
  

The problem is that Canon have never published exactly what
the signals to and from the lenses do, so manufacturers such as Sigma