I went out today and tested the 300+Tamron 1.4x (instead of Canon) converter. The results were the complete opposite! The images were tack sharp. Go into nature/wildlife and look at 300 2.8 IS and see some of the hawks/eagles. There was NO LOSS in image quality with the Tamron Converter. The Canon (my copy I suppose) 1.4x was extremely soft. I am considering going with the Tamron 2x after seeing these results!!!

Thanks a bunch!

make the images a bit bigger, boomgarden.

That happens very often in the sort of perspective you had on those birds. You got them in focus "globally", however with insufficient DoF so that the bird can not be sharp bill to tail. One would perhaps try to focus on the eye, but again, that is a pretty tall order for any camera to do in the said perspective, and even if you accomplished that, the rest of the bird in the foreground would have been even more OoF.

In a nutshell, I think those images need more DoF.
The lens+ TC combo seems sufficiently sharp.
If you are close to the bird and ain't got enough light to stop the lens down sufficiently, then make the bird turn around and face you. Now, in that kind of a shot the head can be easier to focus on properly, and if the rest of the bird blurs in the background, that seems more acceptable to our eyes than having it blur in the foreground.

The Tamron converter was significantly sharper than my Canon. I shot using the same DOF & technique with the Tamron and got SHARP images! I must have got a soft copy of the Canon.

I think the converter is fine, the issue is likely very thin DOF at close distances. Try stopping down more for less margin of error.

My father-in-law and I tested the converters on 3 cameras (5D II, Rebel XT, and 1Ds II) with the 300 2.8 IS. The Tamron converter destroyed the Canon in sharpness. The Canon was just simply NOT sharp. From what everyone is saying, the Canon SHOULD be sharp.

baumgarten0712 wrote:
The Tamron converter was significantly sharper than my Canon. I shot using the same DOF & technique with the Tamron and got SHARP images! I must have got a soft copy of the Canon.

Agreed. Assuming your test wasn't flawed in some way, you've got a bad TC. When I got my 300/2.8 I tested six 1.4x and 2x TC's against each other. All of the TC's performed almost identically (I had to squint at 100% to see a difference), except for one 1.4x (a Tamron, ironically), which was so bad it made my 300/2.8 less sharp than my Sigma 70-300.

I say run another test to make sure of your results, and if they agree with your last test, ditch the Canon 1.4x and pick up a replacement TC. You might also consider other brands if you don't need the weather sealing. I like my Kenko, because its non-protruding front element means I can use it on other lenses, like my 100/2.8 macro. It's also plenty sturdy to use with my 300/2.8 handheld. (I can't say the same about the Kenko 2x... that felt too loose for my comfort.)

The 300 f/2.8 is extremely sharp with the 1.4x II TC even wide open. If you can't get sharp results check the TC or check to make sure the focus is correct.

I use a Kenko 1.4
Almost identical in IQ to the Canon (if you notice a difference, you're pixel peeping to 300%!), and a lot cheaper.

Karl, it's looks like you've got a bad 1.4x II...happens.