I just wanted to post some observations that I've been making while scanning a mountain of negatives and prints for a book project.
It seems that the prints are all resolution limited by the paper surface itself. Beyond 300 or 350dpi, it seems impossible to extract any more detail from the prints. However, larger prints of the same image exhibit dramatically more detail. In either case, the resolution limit is not obvious with the naked eye, but it is evident with a loupe or with a scan that's zoomed in on in photoshop.
Example: I have an image that is printed at my normal size of 11.5 x 9, and also in mural size at 48 x 38 inches. The negative is 4x5, apx 100, very sharp. The mural was printed with an HK horizontal 8x10 enlarger with a modern 300mm rodenstock lens and a vacuum easel; the small print was printed normally with a schneider apo componon hm lens. These are both modern lenses in well aligned enlargers, and can be expected to offer comperable performance. If anything, the small lens should be better as it is half the focal length (made for the format being used) and an apo lens.
In the mural, there's a blackboard clearly visible inside a partially demolished building that has bowling scores written in chalk ... "individual records, 1932" and a bunch of numbers. On the small print, no matter what the resolution of the scan, or the power of a loupe, the writing is an illegible blob. Much beyond 300dpi, there's no additional revelation of detail, beyond those of thee paper surface itself. This corresponds to around 700 to 900 dpi of useable resolution on the negative (which might be defined as resolution that can be rendered at more than 20% or so contrast on an MTF chart). In the mural, however, the letters were so so crisply renered that they have edges on them. I measured cleanly resolved writing at 1.5lp/mm, which must have preserved at least 50% contrast. This translates to 38lp/mm at a respectable contrast, an equivalent of 3600 dpi of EASILY useable resolution on the negative. Which is about 4 times what was available on the print that was ... one fourth the size. If anything things look better for the big print, which has two possible advantages: smoother paper surface (very glossy RC paper, vs. air dried fiber for the small one), and a larger magnification which uses the lens closer to what it's optimized for (my apo componon is optimized for 10x enlargements, but I'm using at about 3x).
So? I think this is helpful information if you're scanning for any kind of printed project. Resolution beyond 300 or 400 dpi at final print size will not be visible, because the paper surface is limiting.
A related point: if paper were not a limiting factor, the next limit is human vision. I learned from a tech rep at Schneider that human eyes are limited to resolving 11 lp/mm at the best viewing distance for fine detail (12"). This has to do with the density of light-receiving rods and cones in our retinas, so it cannot be improved optically with glasses, etc.. This translates to close 558 dpi of resolution, or close to twice the paper limit, if your eyes are perfect. However, this is DETECTABLE resolution. Meaning we might be able to see 11 lp/mm details if they are rendered at close to 100% contrast, and yet we would probably perceive them at a ghostly 5% resolution (about the lowest contrast that's visible).
The Schneider rep told me that the important number is 5 lp/mm--this is the resolution range that our eyes use to perceive sharpness. Which means that having strong contrast at 5 lp/mm (around 250 dpi) at print size maters much more in our perception of clarity and sharpness than the rendering invisible or barely visible details at higher resolutions.