It looks like you're new here. If you want to get involved, click one of these buttons!
That is interesting- your D5100 allows you to set it to 3D. My D800 won't, even with the TC1.7! To quote an old saying, what has yours got that mine hasn't? I.
1. Converters that physically are incompatible with various lenses. ie. as Msmoto says above, the elements of TC and lens actually comes into contact. Obviously disastrous!
2. TCs that do not auto-focus in any, or some, AF modes with various lens/body combinations.
3. TCs that meet the above criteria but which produce unacceptable IQ and are therefore not worth using.
I dislike teleconverters intensely. I would much rather use a DX 7100 than add a 1.4 to an FX camera. DX is essentially an optically perfect 1.5x teleconverter that doesn't cause you to lose a stop of light. If you're using FX, getting a D7100 as a teleconverter is a pretty good deal. If you're already using DX, never mind.