Originally posted by j blair Any guidance? I thought maybe there might be a 250 or 300 mm prime lens that might allow the autofocus to work. But haven't found one yet.
300mm f/4 should work in decent light. But by the time you buy one of those plus the TC you are in the 50-500mm price range again. Unless you can go manual focus, in which case you can get an older 300mm f/4 for not ridiculous money. I have used an S-M-C Takumar 300mm f/4 with a TC. Image quality is OK but lots of CA. And you need really good light.
You don't mention which TC you bought (and there are many different ones that look similar) but if it is a manual one then did you turn on "Use Aperture Ring" in the camera settings? If you did not that can cause the aperture indicator to blink "F---".