Originally posted by tripodquest
See my follow-up question above and please answer it if you can. I really want to understand all these things better, understand why my thinking is backwards. It doesn't seem backwards to me so I need someone to help me understand how and why it's backwards.
When I get back I will try to explain if someone hasn't already done it. There are a lot of points you can address with your question.
The simple answer is that APS-C lenses need to be sharper at wider apertures and faster in general to match the image quality of FF. To match FF you have to shoot at a wider aperture which means the lens needs to be designed for peak sharpness at a wider aperture. The Sigma 18-35 F/1.8 is the FF equivalent of a 27-53mm F/2.8. So the 18-35 has to be as sharp at F/1.8 as a 24-70 does at F/2.8.
There is a lot more too it, but that is one key reason.