Originally posted by fgaudet If your lens sweet spot is f/8, it'll stay at f/8 (on the lens), TC or not. So to keep the same sharpness, with a 2xTC, your lens will still need to be at f/8 but you will expose at f/16.
I have to disagree a bit.
In my experience with, e.g., a 2x teleconverter, stopping down another stop will generally be slightly more sharp, assuming a completely rigid setup (hence no shutter-speed complications). So if the base lens is best at F/8, generally you could pick up a bit of resolution by going to F/11.
It's usually not much though, but with an 2x T/C, every tiny bit really helps.