Originally posted by gazonk Huh? The point is that with FF, you can get better high ISO and narrower DoF without the fastest glass! For instance, the M-40/2.8 will give you narrower DoF @2.8 on FF than the 28/2.8 will give you on APS-C.
Please, follow the advice given in my post you just cited.
To address your particular comment: An FF 40mm/2.8 and an APSC 27mm/1.9 are equivalent lenses, giving same DoF and same low light performance. But the 27/1.9 is NOT faster glass than the 40/2.8: Both lenses have identical aperture diameters and should have equal manufacturing cost.
Except that the 27mm already needs a retrofocus design with a FF mount like the K mount. Which was my point actually, the choice of available glass.
Maybe, we actually want to say the same: i.e., that a FF camera needs more moderate F-stop figures for equal performance. But I wouldn't call it slower glass as this would imply lower cost while in fact cost is equal (except for wide angle and apertures below f/2).