To jest całkowicie normalne. AF w 5D działa do max. przysłony f/5.6
100-400 ma światło 4.5-5.6; po dopięciu TCx2 robi się z tego f/9.5-11 więc to normalne, że autofocus ma za ciemno.
EDIT: mozesz spróbować go oszukać :-)
http://www.fredmiranda.com/TipsPage/
1) How to activate auto focus (AF), when using 1.4x or 2.0x teleconverters with f/5.6 or slower lenses.
It is simple. Just place a small transparent piece of tape on the last 3 pins of the converter. The tape should be placed on the left hand side of the converter when looking at it from the lens connection side. With these three left pins covered, the converter continues to operate on the D30 with the AF activated. It works great with the Canon 100-400mm f/4.5-5.6L IS.
see a picture of the pins taped.
http://www.fredmiranda.com/TipsPage/images/tapemod.jpg