[sjwright]

In terms of equivalence, you're just mathematically wrong.

To demonstrate why you're also wrong with overall quality, consider the Canon 5Ds/5DsR. An incredible 50-something megapixels; famous for being able to reveal flaws in (or at least exceed the limits of) some of the most expensive glass. This is because its pixel pitch of 4.14µm is so incredibly fine.

The latest APS-C bodies, the 80D for example, have a pixel pitch of 3.7µm, which is even finer still. Using full frame glass on an APS-C body is like putting it on a hypothetical 5Ds Mark II and cropping into the middle 40% in post. It's just not going to be sharp.

This is because when you put that expensive full frame lens onto an APS-C body, you're only using half of the actual physical glass you paid for. The rest of it is going to waste. You're just using the middle bit... then magnifying it to accentuate the flaws.

[sjwright]

And just so it's abundantly clear, I am not saying that APS-C is inferior because it absolutely isn't. Nothing is stopping lens manufacturers from making similarly good lenses for a smaller image circle.

The issue entirely boils down to using lenses optimised for one sensor size (in this case full frame) on a camera that has a significantly smaller sensor (in this case APS-C).

[neom]

http://cpn.canon-europe.com/content/education/infobank/lense...

Also, I went through 4 or 5 85 1.4s to find a sharp one because the built quality was so bad, and even after a couple of weeks traveling it had gone soft, never found this with my 50 1.2 (although I ended up ditching all the crop gear and going to FF when I bought the 85 1.2) - However, it was a really good substitute till I could afford it (as I mentioned above) :)