Deciding what resolution you should use when scanning negatives and slides might be difficult, but if you have a relatively fast scanner, a decent computer and hard disk space, a simple answer exists.
If you want a straight to the point, no fuzz answer, use 3600 dpi when scanning a film frame.
This resolution gives you a Decent scanning speed, Very good quality and a File size you can handle.
Scanning speed
The actual scan speed per film frame depends almost entirely on your scanner hardware, and not your computer speed. A low, to medium prized consumer scanner as of 2009 (e.g. a Plustek OpticFilm7600i, priced at around USD 290), will scan a single frame and store it to a JPG file in 40-50 seconds.
The Quality
I spent a long time researching and testing scans on different films before ending up using 3600 dpi as my standard setting. It can be tempting to lower the dpi setting to get a faster scanning speed, but there is a downside to that. I would occasionally find pixelization/jagged edges when magnifying the photo, even on ASA 200 negative films (which is the film type I historically have used the most).
Instead of making a decision on what resolution I should use on each scan, it is much more convenient to just set it at a level high enough to make feel comfortable at all times.
If I keep changing the resolution for each and every photo frame, there is one more tweak to remember - and honestly, I will just forget to adjust it when I really do need it!
I also have the GANE filter activated in my SilverFast scanner software, when scanning at this resolution. This will reduce any grains resulting from visible chemical film material that might show up.
The final digitized picture will be ca. 5000x3200 pixels.
The File Size
When I save to jpg format, as I normally do, a scan like this will result in a file of between 2.5MB - 10MB depending on jpg compression setting (max 10-20% loss, i.e. "High Quality" jpeg)
This is a comfortable file size for most people and is on the same level as a similar photo taken with a modern digital 15 megapixel SLR.
End of story?
There are times when 3000-4000 dpi scanning resolution is a total overkill. I found a film that I used with a single-use underwater camera, probably equipped with a cheap 800-1000ASA film. I gather it would probably be good enough with a 1200 dpi for this one. The end result was actually better than a scan at higher resolution.
After I some experience, it has become more natural for me to adjust the resolution for whole film rolls, and even for individual film frames now and then. Not only do I do this occasionally for bad quality fast films (I do not have many of those), but also for things like low light photos.
The point is - I feel I cannot really go wrong with the 3500 - 4000 dpi setting and it is more than good enough for standard consumer negative films (100, 200, 400, 800 ASA).
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