[Rob Barron]

If you are using a TC with an L series lens, you will definitely lose quality with anything other than the Canon TC as these are optically matched for use with the L series lenses. That doesn't mean the Kenco is a bad TC, it just isn't optically matched to this set of lenses. Yes we're talking about the sharpest areas of detail and to some eyes little difference will be seen but if you are doing detailed work or major enlargements, the differences are definitely there. I do a lot of printing at 18" x 12" and so the difference would be noticeable.

Just to point out though, the Canon TCs are designed for a specific group of lenses, not for all Canon lenses. You can't just take a Canon 75-300mm lens and double it with a Canon 2x TC anyway so a non-Canon TC would be required. Why? Because as I said before, the optics on these TCs are L series quality. There is no more point putting a non L series lens on one than there is putting an L series on a cheaper non-Canon TC.

The design is such that other lenses do not fit on because of the lack of depth of the rear element.

I have to say, if you really want quality, the best way is to get a 300mm prime lens and then put a TC on it. Expensive yes but the quality will be superb.

Cheers,
Rob