Samuel Dilworth , Feb 24, 2001; 06:31 p.m.
Its nice to come across, now and again, someone on the web who actually knows what the terms dpi, resolution, etc. mean. Digital imaging is widely misunderstood on the net, to the extent that at least 90% of sites asserting the superiority of one product over another come up with findings wildly out of synch with truth and reality.
Nevertheless, I see at least one flaw in your reasoning. You state the following:
"When scanning such a negative it takes a minimum of 2 "dots" or samples to resolve a line-pair. Under certain circumstances it can actually take more, depending on the test pattern, but in practice this is a good rule-of-thumb."
No doubt it is a good rule-of-thumb, but if youre aiming higher, its not very useful. If you scan a vertical or horizontal "line" (which obviously has width on the negative, if not in mathematical theory) of one pixel width (of the scanning device), which happens to have a centre in the division between two CCDs, the two CCDs output equal values (i.e. no contrast) so the line is not resolved. (Forget diagonal lines, which require higher resolutions again). By increasing the resolution of the scanning device, you can reduce the minimum width of the line which can be resolved, but in theory you will NEVER be able to capture all the information on the negative (in practice you can get as close as makes no difference with a resolution of 10000 dpi or so with typical slide films like Provia F). Note that your "certain circumstances" in the phrase I quoted here is of course the rule rather than the exception, but 4 "dots" would resolve the line-pair with acceptable dependability. Nonetheless, you do recognize and state that it is only a rule-of-thumb, so I do not have a problem with this (especially considering it eliminates one more variable for the dummies who cant be bothered to learn for themselves).
However, despite your saying so, you come to the conclusion that, because sending more than 350dpi to the printer isnt going to increase actual spatial resolution, there is no point in scanning at higher than the scaled up resolution (with anticipated print size taken into account). This is wrong.
According to your findings, when printing an approx. 11 inch image using the full length of the 35mm negative (36mm), you will not see benefits from a scanning resolution of above 2700 dpi. Well, get a negative with plenty of detail (areas with over 100 lp/mm for example), and scan it at 2700 dpi and have the same negative commercially scanned at over 5000 dpi. Print both resulting files to the same printer (which is capable of a spatial resolution of 350 dpi). View the results, ignoring the obvious differences in shadow detail, etc. and concentrate on just the resolution on the prints. The print obtained from the scan at double resolution will contain more detail (not double the detail of course). Ive tried something similar myself to satiate my own curiosity.
The long and short? Scan at the highest resolution you can possibly afford (higher resolution also increases perceived contrast), and print at the highest resolution your printer can cope with. Dont kid yourself that your cheap scanner is getting you optimum results because it can scan at a resolution "high enough" for the printer, based on the conclusions on Peter Nelsons web page.