[Kombisaurus]

Adam, I took a few test shots with the Kenko Teleplus Pro 300 1.4x today and here are the results:

Firstly, a comparison shot to see the magnification difference the 1.4 teleconverter makes. Both of these shots were taken from the same spot with the EF 50mm f/1.8 lens @ f/2.5 (which was wide-open with the teleconverter). Can you pick which one I used the teleconverter with?



For the next test I used my 70-200 f/4L lens (running at f/5.6 so I could use the same aperture with and without the teleconverter). In this scenario I positioned the camera on a tripod and took one shot at 100mm without the teleconverter, then the same shot at 70mm with the teleconverter to give the same equivalent focal length (70 x 1.4 = 98mm). This image shows 100% crops of the centre of the image, with a little inset pic showing what the whole photo looked like. Can you see any difference between the shots? The only differences I can see are very very minor, and could be accounted for by slight vibrations through the tripod or differences in focus. I was very impressed with these results.



And my final test was using my 135mm f/2L lens (at f/2.8 to ensure aperture consistency). Again I used the same subject and a tripod to shoot with, but this time I took one shot with the teleconverter fitted, then removed it and moved the camera closer to the subject until the subject was back at the same size in the frame, and took another shot without the teleconverter.
The results in the centre of the frame were very similar to the shot above, but this time I decided to take a closer look at the edge of the frame to see if there were differences. In this case the teleconverter does degrade the image quality noticably at the edges, but they are still perfectly usable.



Remember these are 100% crops (the inset shows the whole shot), and to be honest this corner example with the teleconverter is still noticably better than the corners of my EF-S 17-85 when it is at a fairly wide focal length.

So is it better than the Canon teleconverter? Well.. I've never used the Canon converters so I'm stabbing in the dark a bit here...
But I'd expect the Canon to have better quality at the edges, but really I can't see how the Canon could improve much in the centre of the frame because the Kenko seems extremely good there.
The Kenko also scores points for being usable with lenses that the Canon and Sigma won't work with... It allows the camera to attempt to autofocus beyond f/5.6 (although there is no guarantee it will actually lock focus), and big points for costing less than half the price of the Canon and is cheaper than the Sigma too.

I *think* these same teleconverters are sold as Tamrons too... just under a different badge.
(http://www.amazon.com/gp/product/B000068...oding=UTF8) :/ I can't confirm that though.