Originally posted by philbaum . I wrote Tamron and they replied that the Tamron 1.4TC was not recommended for the Tamron 18-250. I find i could still autofocus with the 1.4 and the 18-250, in most cases -1 stop loss of light. But often i disliked losing the stop of light so don't use that combo anymore. You can manual focus with the 1.4 in place, it just doesn't have that silky smooth action that one expects of a Pentax manual focus lens.
Don't have any 2x TC so can't comment on that.
I think Tamron gave you a very sensible response: teleconverters are simply not made to be used with slow super-zooms.
Originally tcs were made for specific lenses only (nothing else, but the ancient covertible lenses). Later manufacturers always stated, that they were intended for use with fixed focal length lenses and over a very long period they explicitly stated, not to use tcs with zoom lenses at all.
Then the first dedicated tcs for specific zoom lenses came onto the market (Vivitar was among the first with the matched tc for the 70-210/3.8 if I remember correctly) ans somehow many people simply thought, that now tcs could be used with any lens, prime or zoom, fast or slow. That is wrong.
The tc will multiply not only the focal length by its facor, it will obviously also enlarge all the inherent flaws of the lens by the same factor. Add to that the unavoidable loss of contrast and sharpness, which even a matched tc for a certain lens needs to produce.
So we can easily see, that a mediocre lens (and all super-zooms are nice, but mediocre from a pure iage quality point of view) becomes a very poor lens, when a tc is added - and it is for good reason, that manufactureres do not recommend the use of these lenses with any tc at all. It might be feasible, but then indeed, cropping might be the better choice.
I don't theorize on this topic, I have roughly 10 tcs (among them all the Pentax modells and the two Sigma Apos) at hand and use them only very rarely and with a few select lenses, where their useage makes sense.
Ben