I tried taking the istD out birding this weekend and I found that it's just too hard to focus manually at an effective 1200mm, so now I'm looking for a solution. The first thing I want to get is an AF teleconverter. I looked at B&H and there seems to be quite a range in prices. Tamron's 1.4 is just $80, but Christian's Sigma EX 1.4 is $169, and on the website it says it only works with certain lenses, none of which I have (and I have a fair amount of Sigma glass). Tamron's 2x has no such restrictions listed and it's $144. Do I have to be careful when I buy an AF teleconverter or is this just a marketing ploy by Sigma?
Thanks, Amita
