I'd like your opinion on an idea I had for a lens collection. I have a Canon 40D and the tamron 17-50 f2.8. I was thinking of getting the new Tammy 70-200 f2.8 (I love fast glas, and from I've read this is a pretty decent lens), as well as a 2x teleconverter. This will give me the equivilent of: 17-50 with the wide angle zoom by itself. 36-100 with the wide angle zoom combined with the 2x TC. 70-200 with the telephoto zoom by itself 140-400 with the telephoto zoom and the 2x TC. I'll lose a few stops with the TC, but considering that the glass is fast to begin with I don't see this as a probem. According to THIS page, I'll lose two stops. Starting with a maximum aperture of f2.8, that still gives me f4, which isn't too bad. The advantages I see are: it gives me the 17-400 zoom range with just two lenses and a TC (27-640 efl), and that's the same as a 23 times zoom if it was in one lens, so I've got a huge range to play with I'll have pretty fast apertures over the whole range I won't have any gaps between the focal ranges. In fact, I'll be doubling up on focal lengths between 36-50mm, 70-100mm and 140-200mm. My questions are: Will the image quality I get from such a set up be good? How much will I lose by using a teleconverter? And which teleconverter should I get to make sure it works with the Tamron lenses, or will any TC that will work with Canon lenses work on the Tammy's (seeing as they are the same mounts)?