Don't know about Sony cameras or the Sigma 70-300 lens Andrew but in general 2x tc's degrade the image quality a lot. It is only on the very best quality prime lenses that you can get away with a 2x tc (300 f2.8, 500 f4, 600 f4 etc.)
A 1.4 tc would be a better bet but even then I would not like to say how good it would be with your 70-300 (I know on the Canon 70-300 any tc is poor and you lose AF).
Hopefully someone with the same combo as you can give you some advise but until then I would be very weary about spending any cash on a tc - just my 2p worth.
|