Originally Posted by olifaunt
This is all speculation:
I think bad scanning complicates things. The Capitol film picture posted here seems like a bad scan so it is not helpful. At that size you really shouldn't see any visible grain in a good scan of hp5-400 film, which I assume it is. My normally exposed and developed trix-400 doesn't show grain viewed at that size; I have to magnify it to see grain.
One big difference with digital I find is that, for example, in negative film there is always detail in whites. By this I mean very slight tonality variation even in "blown-out" areas. If you magnify a good scan you can see it not as grain itself as such, but rather as slightly larger textures comprised of slight variations in grain density, which gives slight tone variation at a larger scale than the grain. One doesn't usually pick up these variations consciously (it might just look like even white) but it jumps out to me when it is lacking, as in blown areas of digital photos, or even non-blown smooth areas.
If I look carefully with my eye at the clearest blue sky, for example, my brain might smooth it over (nothing to see here folks) but if I pay attention to what my eye actually sees, is not even, there are slight local random tonal variations. Some of it may well be small variations in moisture and air density. Some of these variations are probably real effects of local variation in optical properties of the air - this is after all why stars flicker at night and the best telescopes have to be put in space or very high elevations. But part of it is also noise in the visual system (look at a white screen background and at least I see a slight dancing blotchiness of superimposed visual noise). Digital smoothing usually gets rid of this, but why doesn't the eye add natural-looking visual noise for smoothed areas in digital pictures? I think the eye does add this noise, but the visual noise is too large a scale to look natural for a screen-sized picture, so a photo of the sky looks too smooth. Turning off noise reduction in digital will often help a little for this problem.
As far as the grain in the capital building why would the scanning make it that much more prevalent? I haven't noticed big differences in grain between my scanners though I haven't really gone looking for that either. I find bigger differences in grain based on developing than scanning. Or it could be processing of the scan... sharpening making it stand out.
As far as tonal variations in white areas I think that is a couple of things. First, obviously film handles highlights very differently than digital if you overexpose. If you clip digital it is gone. Second, tonal curves are pretty different between the two. There are processing options that can change the look of a non-clipped digital file in the highlights by changing the tonal curve or adding contrast to the highlights.
Lastly, I think digital files just tend to be brighter than film overall which changes the proportions a little. Underexpose digital a little and leave more highlight room and files tend to look more 'organic.' One of the things the Acros mode in Fuji's seem to do is meter a little differently and leave more room in the highlights. Likewise shooting a Fuji in forced DR400 mode has a tone curve that seems a little closer to film.