if your net aperture after adding 1 stop for the 1.4 converter or 2 stops for the 2X converter is slower than f5.6, the camera bodies will pretty much not be able to achieve focus lock. that means that you really should be under f2.8 for your base lens and f3.5 as the slowest where you have a reasonable chance of AF actually working. an f4 lens will be really hit and miss. shame about the Sigmas. i just sold both my 1.4X and 2X Sigma EX AF extenders today.
Herb... ----- Original Message ----- From: "Amita Guha" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, May 31, 2004 5:12 PM Subject: AF teleconverters > 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?
