Hi Michael,
I can't comment too specifically on the Tamron-F AF teleconverter 1.4X Pz-AF MC4 yet as I have previously used Kenko. I have a Tamron 1.4x on order so I may be able to say a little more when it arrives. However, from using the Kenko TCs I can comment in general. As I understand it the F4.6 which is mentioned is stating that in order to get autofocus you need the main lens to have an open aperture of F4.6 or brighter. For example my old Kenko 1.5x required an open aperture of F4.5 in the main lens. I was able to use it with my DA*300 which had an open aperture of F4.0 and get autofocus through the aperture range. In principle, as far as I understand, using auto exposure the TC should work whatever aperture you want to take the picture at because the aperture will be fully open until you press the shutter. Any TC is going to be more useful on a good prime rather than zoom lens, but I have seen some acceptable images taken with a zoom/TC combination. The main issue that I find is the loss of the 1 stop, especially in England, where light (photography wise) is not always good. It's even more of an issue as I like to do some wildlife photography which means I also want higher shutter speeds (which are desirable using a lens/TC combination anyway in order to stand the best chance of getting sharp images). There are a lot more people on the forum with more expert knowledge of TCs than myself so I am sure there will be others who can comment. There are also some other threads that may be useful if you do a search.
Paul
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