Originally Posted by audiobomber
The test photos I've seen with a TC and Sigma and Tamron 70-300mm and Pentax 55-300mm telezooms were disappointing, i.e. inferior results to no TC and cropped.
I can only speak about the Tamron 1.4x teleconverter and the Tamron 70-300 LD Di. With that combination, results depend on variables that I have never really nailed down. Basically, though, I think if the light's good and I can get a good exposure with a fast shutter and without having to open the lens up all the way, TC + lens is an improvement over lens alone with cropping in post. Or if it's not an "improvement" in image quality, at least, under good conditions, it's not noticeably worse. The TC of course costs me a stop. But I've taken a ton of photos with the converter on and with it off, and I know I've taken photos I was pretty happy with both ways. Unfortunately, the EXIF info doesn't tell me if the teleconverter was used or not. (One of my biggest gripes about using the TC.) The lack of EXIF info makes it hard for me to compare photos in retrospect and I don't have time to do any tests right now. On the other hand, I can tell you that when I review my photos, I can't tell from image quality alone if the TC was used or not, which I guess is worth something.
Will