Since a lot of you are digital camera people or doing photo restoration on old snapshots I think a short (very short) note on film resolution might help. I’ll stick to 35 mm since that was the most popular film for a long time. The information can be adjusted for larger or smaller formats.
Quality lenses are able to resolve parallel black and white lines down to at least 40 per millimeter. That means that there are eighty "pixels" in a millimeter – 40 black and 40 white. This is definitely a floor as most are at 50 lines/mm and above. Standard lenses (i.e. the 50 mm) and the more expensive lenses can go much higher, particularly Leica and Carl Zeiss lenses.
Film, with the exception of very early high speed (ISO 400+) film, can produce images at 40 lines per millimeter.
Now if you take that information along with the 24×36 mm format of the film, mix with the 3.5×5 size of the classis snapshot and you get a print that’s at least 550 "pixels" for resolution. 50 lines/mm would be roughly 685 "pixels". Use of quality lenses and slow film can double and sometimes triple that. So people who use 600 dpi as their upper limit may be just fine for most of their prints.
All of the above can change dramatically with a smaller or larger negative. A 120 film negative (Bronica, Hasselblad) is 60 mm x 60 mm. Which is a minimum of 4800 x 4800 "pixels" although most 120 film cameras have superb lenses and are more likely to be at least 7000 "pixels". Depending on the print size 600 dpi scans may be enough but usually not. A snapshot taken with an old plastic lensed 126 camera would, of course, be much lower and the image would be "soft".
Really old prints are often contact prints meaning the print is the same size as the negative. Those negatives were very often glass 4×5, 5×7, 8×10, or some odd size and the photographer used very slow (translated=high resolution) lenses. Such prints are often called museum quality and will test the scanning limits of any system.
The above applies equally to B&W and color print. Color tranparencies are much more complicated because of the multiple layers, aging (worse than color negatives), "blooming" which refers to the spreading out of a dye and finally the color fringing of lenses. Old slides can get pretty bad but more recent ones will definitely be helped by scanning them with a film scanner rather than a slide adapter on a flat bed scanner.
wrote in message
I’ve been given a photo that was made in 1900, it is 6" x 7 1/2", faded,
and very weathered, it has not been in a frame, so it is covered with wear marks. I’ve repaired a few old photos using PE2, but I consider myself a beginner, and when I scanned this, and saw it on the monitor there was really no good areas to clone etc. It is the entire family of my gg grandparents, and I would love to have it repaired. Is there somewhere that I can send a scanned copy to see if it is repairable?