This may seem a silly question.
I use the 300 F4 with an EOS 1D Mk 11, without the 1.4 x it produces fine and sharp images. With the 1.4 x the auto focus does not work very well and everyting is slightly blurred. Do I need to set up for aperture priority using a maximium of F8?
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In general, the more light the AF sensors get, the better they're going to work. AF happens with the lens "wide-open" no matter what your settings, so the camera should focus as fast with that combination of gear at Av f/5.6 (f/4, minus a stop for the TC) as it would at Av f/22. But I would expect it to be a bit slower than without the TC.
There are all sorts of reasons for blur. Start by making sure the optics are okay, do a set up at home, with a tripod, cable release, mirror lockup, fixed easy to focus-on subject, etc. Try it with and without the TC, a Canon TC 1.4x II shouldn't make that much of a difference in sharpness on the 300L/4. If you pixel peep I wouldn't be surprised if you see *some* difference, but for that TC it shouldn't be much. Don't forget to have IS off for this test, if you have that version of the 300L/4. Make sure you're far enough away that the camera can actually focus on the subject, too.
If everything is a lot blurrier with the TC under those controlled conditions, I'd suspect the TC. Otherwise, I'd suspect you're seeing some combination of camera shake (as you add focal length it gets harder and harder to keep the camera steady enough), subject blur if you're dealing with wildlife, and so on. There are some good articles on long lens technique out there to take a look at.
