Originally posted by falconeye My FA* 300/4.5 + Tamron 1.4x deliver flawless AF performance on my K20D. It is not as snappy as w/o the converter. But it works. Looks like the limit is more like f/6.3 rather than f/5.6.
Actually, the limit is more like f8 (effective max aperture) for AF in the Pentax bodies. I use my FA* 300/4.5 with the 1.7x AFA (effective f7.7 max aperture) quite a lot, and it will AF reliably on all but the darkest overcast days. On the other hand, my Tokina 150-500 f5.6 AT-X SD would not AF with the AFA at noon on June 21 on a cloudless sunny day as it's just too slow (but I was hoping . . .)
Scott