A buyer's guide to the Canon EOS lens system

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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.


  • 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 & Tamron have had to work them out. With non Canon lenses on newer bodies, sometimes there are extra commands that the lens doesn't understand, sometimes they aren't important, sometimes they prevent the lens from working. Older Sigma lenses in particular have problems on the Digital bodies, some work on the D30 (not the 30D!) and D60, but that's all, on newer digital bodies the lens will close the aperture, but not open it again!! You can sometimes upgrade a chip in the lens, but often it's cheaper to buy another lens!
Summary: Canon lenses: OK. Non Canon: If it doesn't say, then ask!

Lens types

Lenses come in two basic types, known as zooms and primes. Primes have a fixed focal length, zooms have a variable focal length. Because primes need fewer elements than a comparable zoom, for the same overall performance the zoom will need better quality optics, so for the convenience of a zoom expect to pay more (maybe a lot more) than for a similar quality prime lens. Unfortunately the laws of physics prevent you getting something for nothing!

Auto focus types

In the Canon lenses, the original auto focus motors were compilmented with Ultra-sonic ring motors. As the ring motor only engages when they are actually turning, the point of focus can be manually modifed without overloading the motor. This is known as full time manual (aka FTM). Later Canon introduced ultrasonic micro motors, without the full time manual, they are effectively only slightly faster version of the old style motors, but give Canon the chance to call the lens Ultrasonic anyway. Other manufacturers have used similar technologies, but none have really matched the Ultrasonic Ring motor for accuracy and speed.
From best to worst:
  • Ultrasonic ring (with full time manual override)
  • Ultra sonic
  • Plain auto focus


The Canon L (Luxury) series are by far the best lenses money can buy. If you look at any gathering of professionals, those with Canon equipment will be using L series lenses.

If buying non-Canon lenses, it is essential to try before you buy (or at least secure the offer of a refund). Not only are there various compatibility problems, but many non-Canon lenses have front or back focus problems, that vary from one example of a lens to another.
I'm not saying that Canon doesn't have quality control problems, but if you read the forums, then you'll find that Sigma have more problems than most.
There are several signs that a corners are being cut, and that a lens is being built down to a cost, rather than up to a quality level.
There are a few exceptions, but generally if it's got two of following then avoid it:
  • Plastic mount.
  • No distance scale.
  • No USM, or Non-ring-USM.
If you're on a budget, consider the older push pull designs, you'll find for about ½ the price of a modern "cheapie", you'll get a much better set of optics.


So Canon lenses, from best to worst:
  • L - series with USM
  • L - series
  • Metal mount with Ring USM
  • Metal mount with USM
  • Metal mount
  • Plastic mount - non-Ring USM no distance scale
If you've made the decision to carry around an SLR instead of a point and shoot compact, then do it the justice of putting a reasonable lens on the front.

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