[quote name='mst' timestamp='1312797599' post='10492']
That seems to be a limitation of the lens, not the 5D II. It isn't any different on the D7000 or D3x. In fact, it's mentioned in the manual of the lens that it does not provide AF with Sigma's own 2x TC (for whatever reason).
That limitation also existed in the non-OS version (which is why I tested it with the 1.4 TC only).
Sigma generally is able to build f/2.8 lenses that AF with 2x TCs (the 70-200 zooms for example). I can only speculate why this doesn't work with the 150/2.8. Maybe it has to do with the effective aperture (with a 2x TC added, the lens is f/5.6 at large distances only).
-- Markus
[/quote]
f/5.6 is not a hard limit anyway though - most cameras will try to AF beyond if they don't know. However, they might have done it to assure a reasonable AF accuracy.
That seems to be a limitation of the lens, not the 5D II. It isn't any different on the D7000 or D3x. In fact, it's mentioned in the manual of the lens that it does not provide AF with Sigma's own 2x TC (for whatever reason).
That limitation also existed in the non-OS version (which is why I tested it with the 1.4 TC only).
Sigma generally is able to build f/2.8 lenses that AF with 2x TCs (the 70-200 zooms for example). I can only speculate why this doesn't work with the 150/2.8. Maybe it has to do with the effective aperture (with a 2x TC added, the lens is f/5.6 at large distances only).
-- Markus
[/quote]
f/5.6 is not a hard limit anyway though - most cameras will try to AF beyond if they don't know. However, they might have done it to assure a reasonable AF accuracy.