I'm assuming in all this that you may want to enlarge the photo, or crop and enlarge part of the photo. If that is not the case, and you only intend to print the photo at the same size or smaller, simply scan at the resolution you want to print at, and you're done. What the following images are designed to show is how much information we can get out of the original print itself, so that we don't gain anything by scanning at higher resolutions no matter how much we try to enlarge the photo by doing a higher resolution scan.
It became a challenge trying to figure out how to display resolution on the monitor, when the typical browser shows things at lower resolution than you can scan and print at. I solved this in two ways. First, I enlarged the cropped area a ridiculous amount to emphasize the problems to display on the monitor, and second, I printed the cropped sections, and scanned the print. So hopefully between these two methods, I'll get the point across.
Take a look at the scans of the cropped area enlarged to 4x6. After enlargement, the images were resized to 72ppi for display on the monitor. Personally, I don't think it changes much. Beside each image is the scanning resolution I used.
For all scans, I used a bicubic pixel interpolation when I made the enlargement to cut down on pixellation effects. For more on interpolation, what it is and how to get it, check out here.
Looking through the photos, it is obvious that as you increase resolution from 75 to 150 ppi you gain detail, and as you increase from 150 to 300 ppi you still gain some information. From 300 to 600 dpi you might gain a little. Looking at a print with a loupe, I believe at least 95% of the detail (if not 100%) is captured in the 300 ppi scan. The 300 ppi scan looks a little grainier to me than the 600 ppi scan, but that is an issue of interpolation, not of extra detail taken from the original photo. If I were not enlarging to such an exaggerated extent, the scans would be about equivalent.
Going from 600 ppi up, even at this magnitude of enlargement, the scans are equivalent. If you mixed them up and presented them to me without the resolution attached, I could not tell which is which. Even if you do gain a bit extra going from 300 to 600 (which is still debatable), you certainly do not going above 600 ppi. Thus, I do most of my scans at 300 ppi. For the rare very small photo, which I think I may want to enlarge more than usual, I scan at 600 ppi. I do not scan prints above 600 ppi, since I would not get anything extra other than file bloat.
The next page shows a scan of a page printed with these images.
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