Acceptable Quality and Resolution for Digital Photography Archives

My sister has asked me for copies of photographs from about 10 years ago, when she visited my in Italy.  The truth is I'd love to share those pictures but most of them don't exist in any digital format.  She's now looking into some other options for her other old photos but it got me thinking: what's an acceptable "archive" format?  I remember the old days of ripping CDs to MP3s and regretting years later that I didn't have the foresight to rip to a lossless format like FLAC (or even WMV).

Years ago I bought a scanner with the intent of scanning as close to source material as I could (35mm negatives, slides, photos themselves when there is nothing better).  I didn't get very far, which is a shame because I have a lot of cool memories hidden away in there from trips to Russia, Italy, Spain, London, etc...

What is an acceptable resolution for photographs?

According to the U.S. National Archives standards, a digital equivalent should roughly match the quality of a 35mm photo.  That's a good benchmark.

Here are a few of the details for digital photos:

• Color images must be produced in RGB (Red Green Blue) color mode as 24-bit or 48-bit color files.
• Effective January 1, 2005, digital camera files must be captured as 6 megapixel files or greater with a minimum pixel array of 3,000 pixels by 2,000 pixels. Records produced at this resolution and size are comparable in quality to 35-mm film photographs, which is the minimum quality level for still pictures currently accepted by NARA [see 36 CFR 1228.266(b)].

Here's the scoop on scanned photos:

• Photographs must be scanned as minimum 2,000 line files 8 to approximate a 2 megapixel file according to the following image size and resolution guidelines. (Note: image resolution for scanned images of photographs will vary according to the size of the source document, which may affect actual pixel dimensions and aspect ratios):
• Scan an 8" x 10" original (print, slide or negative) at 200 dpi 9 to produce a file that is 1,600 x 2,000 pxels.
• Scan a 4" x 5" original (print, slide or negative) at 400 dpi to produce a file that is 1,600 x 2,000 pixels.
• Scan a 35-mm original (print, slide or negative) at 1400 dpi to produce a file that is 1,300 x 2,000 pixels.