Branden is actually right. In theory. Diffraction has nothing to do with photosites being "buckets" but instead with the size of the aperture relative to focal length and sensor size. And it's the same on film, digital, or whatever else:
http://en.wikipedia.org/wiki/Airy_disc
What he's referring to has nothing to do with diffraction and is instead related to the idea that emulsion is roughly flat and so it will respond to light hitting it at any angle, whereas photosites will reject off-axis light because they're built up like little buckets and if light isn't entering at the right angle it will hit the edge of the bucket, not the light-sensitive part. Fast wide lenses that aren't designed specifically otherwise will have more oblique rays toward the edges, if I'm not mistaken, so the edges should be darker since less light is hitting straight-on.
But buckets don't have micro-lenses and so the comparison isn't entirely true. Otherwise, tilt/shift lenses would produce unacceptable vignetting on digital. It depends largely on photosite design and a million other factors. In some cases in-camera software will correct for or attempt to mask vignetting, too, so it's really hard to say. If you look at medium format digital backs, for instance, some work fine with tech cameras (which are essentially view cameras and which produces lots of off-axis rays) and some won't due to the bucket issue. It's a matter of sensor design, mostly relating to micro-lenses if I'm not mistaken. But yes, in theory digital vignettes worse than film. Film's softer contrast may have a bit to do with it, as you mention, but I think that's only half the story.
http://en.wikipedia.org/wiki/Airy_disc
What he's referring to has nothing to do with diffraction and is instead related to the idea that emulsion is roughly flat and so it will respond to light hitting it at any angle, whereas photosites will reject off-axis light because they're built up like little buckets and if light isn't entering at the right angle it will hit the edge of the bucket, not the light-sensitive part. Fast wide lenses that aren't designed specifically otherwise will have more oblique rays toward the edges, if I'm not mistaken, so the edges should be darker since less light is hitting straight-on.
But buckets don't have micro-lenses and so the comparison isn't entirely true. Otherwise, tilt/shift lenses would produce unacceptable vignetting on digital. It depends largely on photosite design and a million other factors. In some cases in-camera software will correct for or attempt to mask vignetting, too, so it's really hard to say. If you look at medium format digital backs, for instance, some work fine with tech cameras (which are essentially view cameras and which produces lots of off-axis rays) and some won't due to the bucket issue. It's a matter of sensor design, mostly relating to micro-lenses if I'm not mistaken. But yes, in theory digital vignettes worse than film. Film's softer contrast may have a bit to do with it, as you mention, but I think that's only half the story.
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