And so the aperture will change? Doesn't make sense to me. It's a 90mm f/2.8 constant aperture lens. I believe you were talking about internal focusing, not aperture
What lenses? 85mm f/1.8 yes, 50mm f/1.8II yes, and I also tried out my friend's 50 1.4 (until he dropped it and it has a lensbaby effect now, pretty cool!).
None are very sharp wide open. Doesn't mean they're unusable, just not sharp to me. It's not ever going to be 70-200 f/4IS quality wide open.
With that being said, 85mm wide open is better than 50mm 1.8 wide open, which is better than 50 1.4 wide open. If they're all at f/2, the sharpness difference is negligible.
Buy the focal length that you need.
And ombre, just don't think about teleconverters. They won't work.
They work well on 70-200 series, 200 2, 200 2.8, 300 2.8, 300 4, 400 2.8, 400 4, 500 4, 600 4. They "just about work" on 100-400L, 400 5.6, 800 4.
As you can see, you don't need to think about tcons until you spend a sizable amount of money.
Cheers,
Zexun