Thanks for the reply and the info. I have a great copy of the 100-400 which is as sharp as my 400 5.6L at 400 mm f/5.6. My 100-400 even compares well vs. my 500 f4 IS at the center of the frame. I believe my copy of the 100-400 can harness the resolution of the 50D's sensor very well, at least in the central portion of the frame where it matters for my bird photography. This is because I see a good improvement of captured detail on a per frame basis (as opposed to per pixel basis) when using this lens with a 1.4x TC on a 40D, which use effectively doubles the pixels/bird from the same distance.
Here are links to my user report:
1. 100-400 vs 400 5.6L - http://birdphotoph.proboards.com/index.cgi...&thread=352
2. 100-400 compared to my 500 f4 IS - http://birdphotoph.proboards.com/index.cgi...&thread=353
You are very lucky to get a 100-400 which appears to be so good. For a long time I've been a strong advocate of thorough MTF lens testing according to international (ISO) standards. I believe that every lens shipped, for serious photography, should come with extensive MTF charts so the customer knows exactly what he/she is buying.
The alternative is 'cherry picking'. This can be very time-wasting, and also requires a co-operative retailer. In general, it's a very unsatisfactory approach, because those who have the relationships which facilitate such cherry picking, benefit, and those who don't, and who pay the same price for their lens, get the rejects.
However, I'm impressed with the thoroughness of your tests. Can I persuade you to buy a 300/F2.8 IS and do comparisons with the 1.4x and 2x extenders?
Fike makes a very valid point, that the 300/1.4x combination gives you an 420/F4 lens which has faster and more accurate autofocussing than the 100-400 at 400mm. I can only assume, because I can find absolutely no comparisons on the internet between the 300/1.4x and the 100-400 at 400mm, that the differences in absolute resolution are too small to be an issue.
It's a fact of photography that sharp results are often a combination of accurate focussing and high shutter speed, rather than some slight instrinsic resolution advatage of one lens over another.
I know my 100-400 is not as sharp at F5.6 as it is at F8, but the difference is very marginal. The difference in DoF between F5.6 and F8 is far greater. This is where practical considerations come into play and often reveal the irrelevance of pixel-peeping advantages. If focussing is not 'spot on' then a bird's eye at F5.6 might look disturbingly less sharp than at F8. However, it's not always possible to get focussing spot on. Using F5.6, it's more critical to get accurate focussing. Using F2.8 must be a nightmare.
The 50D with its very high resolution LiveView LCD screen, facilitates tremendously accurate manual focussing. At 10x magnification, a 400mm lens becomes a 4,000 mm lens (compared with an actual 4,000mm lens with no magnifiaction on the same LCD screen.) Of course we don't have any real 4,000mm lenses for DSLRs. The Canon 1200/F5.6 costs as much as a house.
There's simply no excuse for misfocussing with the 50D when manual foussing is practicable.
Out of interest, I went back to my 5D/50D comparisons, and selected images comparing the 50D at F5.6 with the 50D at F8, at 400mm. Whilst the the 100-400 is very marginal sharper, in the plane of focus, at F8, the most significant increase in sharpness results from the increased DoF that F8 offers.
Below, I've taken the same scene at F5.6 and at F8, and cropped a vertical strip from the centre to show the variation in DoF. In my opinion, the resolution differences that occur due to DoF are far more significant than resolution differences at the plane of focus. This scene was taken in the late evening and is across a river, from one bank to the other. Atmospheric distortion is minimal. (It's fortunate that my neighbours are not given to prancing around in the nude, otherwise they might be quite concerned about a guy across the river with a telephoto lens ).
The plane of focus is shown in the lower half of the middle image. Above that and below, you can see the loss of reolution in the F5.6 shot due to DoF. At the plane of focus, we're nitpicking.
[attachment=12738:0013_50D...ll_scene.jpg] [attachment=12739:top_of_c...5_6_v_F8.jpg] [attachment=12740:middle_o...5_6_v_F8.jpg] [attachment=12741:bottom_o...5_6_v_F8.jpg]