Scanning and printing resolution

Some rambling based on the data here (actually no link due to Javascript clipboard spam).

They're measuring resolving power in lines per mm (lpmm) which I suspect is just another way of saying dpi when the page is viewed at a standard distance.

They suggest that the resolving power of the human eye is not much more than 8 lpmm; assuming a viewing distance to the page of 30cm, a line at 8 lpmm is being seen at 2400 diameters, and the widely accepted maximum resolution of the human eye is 1 arcminute (1/60 degree) or ~ 2500 diameters. So that corresponds.

They also say that a good lens at optimum aperture would exceed 100 lpmm; at the same viewing distance that's 30,000 diameters or about 7 arcseconds (7/3600 degree). That's rather poorer than the 0.5 arcsec resolution of a 250mm reflector, but of course that's separation of point sources (stars) rather than resolving dark features. So I don't think that undoes the correspondence above.

1 lpmm should be 25 dpi, as we can confirm by reading the assertion that printing has to be at 25-30 lpmm (525-750 dpi) to look sharp, and that corresponds to the 600 dpi of decent laser printing.
(They say that this 25-30 lpmm should theoretically be 10 lpmm - the resolving power of the eye as mentioned above - but in practice low contrast images require more resolution).

Now that we've established that their terms can be translated into our terms, we can tackle the question that made me start this post - 'What is the maximum resolution at which it is worth scanning a (positive print of a) BW photo?'

Their figure of 75-100 lpmm for photographic printing paper suggests 1900-2500 dpi, and for what it's worth that corresponds to the sort of resolution I found useful for pulling fine detail (eg text on papers held by people in the picture) out of old photos I'd scanned.