Why should a APS-C lens be weaker on an APS-C sensor, dave? The center remains the same and as Klaus asked " Do you really need tack sharp corners at f/1.8?" resolution of 7D MkII and D500 are just the same, so the real question is "have you ever missed a high quality fast standard zoom in that range of FL?" and the next question "...and are you willing to carry it's weight around?"
Like with the 18-35/1.8 there is nothing to compete with - as fast and as zoomy, that is. After Sigma's fantastic 50-150/2.8 APO, there's not much choice for fast short tele zooms. There are fixed FLs better than those two zooms - but also no lightweights. To me it appears Sigma is the only manufacturer really caring about APS-C although it's an illusion to get cheap, light and sharp APS-C lenses. They make it like more for their own APS-C/APS-H cameras. And those are more demanding than other products.
Klaus mentioned also a fast AF - so you have a nice fast indoor sport zoom, even concerts or theater lens. In all situations when I have the choice to loose the shot because I cannot change lenses fast enough, this thing would have relaxed my stress. When I look back I remember a lot of situations when such a lens would have been the solution. Together with a second body and the 18-35.
In low light situations nobody will miss cover performance - you even can't focus in the corners with most APS-C DSLRs and the noise will be killing more resolution than the lens does.
I suggest you guys meet and count the pixels with a magnifier. You simply can't be too careful with equivalent areas...
Ok thanks BC, I would use the diagonal as it takes into account both sensor dimensions!
