Originally posted by normhead Have you ever tried the F 1.7x AF Adapter on it? I use it on my Tamron 300 2.8 all the time and it performs very well. That would get you to 510mm ƒ4.5 with limited AF for a fraction the price of a 500mm ƒ4.5 lens.
I didn't think to use the 1.7x adapter mainly because I have been dissatisfied with it (never seems to do well on combo's).
Well, a very quick test in dim light has me curious to test some more. I have combined the A*300/2.8 with 1.4XL and 1.7XF tc's. Image quality was surprisingly good. Also, light transmittance was good enough to allow for AF to function (perhaps this is related to the quality of the A*300mm lens (perhaps the T-stop of the A*300 is better than the average bear).
If the above combo works in broad daylight and the IQ is better than cropping then I have a semi-AF 714mm f5.6? - if so - AWESOME