There may be a few reasons.
Wind can definitively be a problem. You can activate live view on your Canon and check the image at maximum magnification. If it moves around it really indicates that you would have problems. When shooting tripod I always use live view at maximum magnification for focusing and that works as a reminder on stability.
Image stabilisation may also cause some unsharpness. Stopping down causes diffraction, but with f/11 you should be OK. Checking focus may be a good idea. Some haziness may come from air vibration.
Here are some long lens shots:
Bird, low ISO, 70-400 zoom at 400 mm APS-C, 24MPhttp://echophoto.smugmug.com/Travel/Stuff/27603520_zJWRhh#
Osprey, 560 mm on APS-C, 24 MP, windy conditions:http://echophoto.smugmug.com/Travel/Stuff/27603520_zJWRhh#
Bison, 400 mm on full frame 135, handheld:http://echophoto.smugmug.com/Travel/Stuff/27603520_zJWRhh#
Mule deer, 400 mm APS-C 24 MP, tripodhttp://echophoto.smugmug.com/Travel/Stuff/27603520_zJWRhh#
Moose, 400 mm, APS-C 24MP, tripodhttp://echophoto.smugmug.com/Travel/Stuff/27603520_zJWRhh#
I'm not exactly saying those are all sharp, but I guess they are pretty typical of what I achieve.
I've got a long lens conundrum and wondered if anyone might be able to help.
I'm getting unacceptably soft results for 400mm compressed landscapes with Canon 100-400L @ 400mm with 5DII.
But only for longer exposure shots 1-4 second range. Here is an example of the kind of shot I'm taking like this, though its not magnified enough to show the softness:http://quintinlake.photoshelter.com/gallery-image/Western-Desert-Journey-Egypt/G0000kFUwXry.h20/I0000Tvj31swO1LQ/C0000JnIJLx1O284
To try to rectify the problem I bought a Canon 400mm f5.6 prime and while its a better optically, especially at the corners my results are still too soft for 1-4 second exposures (again fine for faster shutter speeds).
Therefore I'm guessing my series 2 gitzo ( GT2541 with RRS BH-40) isn't sturdy enough or perhaps there are are other long lens techniques I'm getting wrong???
As i'm not a wildlife shooter I'm not very familiar with long lens techniques so when i use my 400mm I feel like i'm starting again which is most frustrating. If you have any technique or gear suggestions i'd be most grateful!
These are the technique and gear I've using for my 400mm shots:
Checked lens foot tight
Checked all ball head knobs tight
Checked all tripod adjustments tight
Gitzo GT2541 with RRS BH-40
RRS plate to lens
Using mirror lockup
Using cable release
100 ISO for max detail in landscapes
Taking multiple frames with a few seconds between
Hanging bag from centre hook