Originally posted by dlhawes ......, given two lenses of identical focal length, the apsc lens is going to concentrate the same light in a narrower focus, while the ff lens is going to disperse the same light in a more diffuse pattern, resulting in a loss of analog resolution.
I disagree on the wording here, focus is same (focal length! ) and difuse would mean unsharp...
I think you can look at it like this: an apsc lens can be created from a FF lens by cutting away an outer rim of the lenses, the outer 1/2 (just guessing) cm of the diameter is not needed. The sensor sits on the same place as the FF one, but is smaller and hence diameter can be smaller.
A good FF lens will be good on both sensors.
But we know sharpness into the corners is sometimes a problem on cheaper lenses and complex zoom lenses...
Hence an apsc usage of a FF lens might suffer less from corner unsharpness or vignetting, as it doesn't use the FF corners.
Making larger diameter lens glass is more difficult than smaller diameters. They will be also thicker in the center or edge of the glas (think about the 2 typical shapes of a lens
https://www.kindpng.com/imgv/TRomixm_convex-and-concave-lenses-concave-lens-convex-lens/ ) and thicker/larger lenses are more difficult to produce...
Here the smaller size difference might benefit apsc designs: they are easier to produce because using smaller and thinner lenses. Hence an purposely designed apsc lens design is more compact...as we all see on our apsc lenses (DA series)
Given a certain skillset of designer and poducer of lenses, there is bit more margin, degrees of freedom he can exploit and invest in image quality, hence an apsc lens might be perceived better as FF if build with same production skills or tools. Or he can just produce cheaper for same quality exploiting the size production-advantage.... Hence I would not always assume apsc is always a better lens design, it might be just a cheaper design...
Unfortunately a smaller sensor is more difficult to produce for the same image quality... Otherwise we would all use smartphones by now? So the various sensor sizes have justification, but advances in sensor technology allow more and more the smaller sensors... this market shift to smaller sensors and smaller, cheaper lenses is ongoing for some time and existed also in film. The desired quality level is the bottom. Oh, and digital postprocessing, AI or not, is a new factor pushing the limits, it's not only sensor hardware.