Hopefully not a really dumb technical question

[jacobovs]

I don't use TC's very often so I have an ignorant question. I noticed that most of my lenses have a sweet spot aperture. If the sweet spot for the 200mm f2 is around f4 does that change when I put a 1.7x TC on the lens. Does it move up by 1.5 because of the TC?

[SmartWombat]

With Canon, the 1.4TC has only 1 stop of exposure difference - and yes, the sweet spot stays where it is, but because of the loss in the TC it appears to the camera it's moved 1 stop from f/5.6 to f/8.
So if your converter loss is 1.5 stops, it's going to move from f/4 to f/6.7 sh.