>>3360696It's not SUPER UBER common but it's common enough that most of sigma's art lenses display such tendency, and more realistically speaking, most high range super fast lenses today show virtually no real world improvement past the first stop down (like, f/1.4 to f/2) because they're meant to be excellent wide open and they hit diffraction from f/4 to 5.6 and the same goes for other slower lenses, which usually show better corner recovery but, again, the corners are being made to be usable at 50mp wide open, so if you're shooting a 30mp or lower body you won't see a big difference if you're not pixel peeping.
This is because with 30+mp count bodies and improving lens technology you just can't sell soft lenses wide open, Sigma would have a monopoly on the market if CaNikon, but especially Canon, didn't follow suit.
