"The 90D is a high megapixel crop sensor camera, and as such has among the smallest pixel pitch of any Canon camera.
It basically boiled down to the fact that the 90D can exceed the capability of some shooters and lenses."
John is correct. When you get in to specs and ability that a camera like the 90D everything has to be on its A game. It is diffidently not a "point and shoot" camera if you are looking for the best IQ. The EF-S 17-55mm f/2.8 IS USM Lens is a wonderful lens. It is the best Canon has to offer in this category. Best fact is, it is a constant f2.8 aperture. The Sigma 18-35mm f/1.8 DC HSM Art Lens is sharper, no doubt. Also a constant aperture lens at f1.8.
However, since you have already ordered the EF-S 17-55mm f/2.8 IS USM Lens, I and you will be happy using it. Perhaps you do give up a bit of IQ but you gain 20mm on the long end. That's worht something and will make it more versitile.
If you still feel you want or need the sharpest photos possible I would buy the Sigma 35mm f/1.4 DG HSM Art (prime) Lens. It just might be the sharpest lens on the planet for a DSLR like the 90D. Again, it is diffidently not a "point and shoot" camera if you are looking for the best out of it. Do your part and it will do its part.
EB
EOS 1DX and 1D Mk IV and less lenses then before!
