Originally Posted by Jim the Obscure
My question is (& I've searched a lot) what is the optimum PPI resolution for scans of b&w photographs to be imported into Sigil for display on both e-readers and in something like Calibre on a computer? ...
In actuality, there are two answers.
First, to ensure that you get the best results you should ALWAYS
scan photographic prints at something in the range of 300-600ppi. Film should be scanned at much higher resolutions; 35mm at around 3200-6400ppi, 120 at around 1800-3200ppi. This PPI is the scanning resolution and is measured relative to the physical original placed in the scanner. These files should be saved in an 8bpp color format that does not use a lossy compression method (read: avoid JPEG like the plague and don't use PNG unless you really really know what you are doing and know your scanning application EXTREMELY
well). You can scan directly into a good image editing app to avoid saving a disk file at this point, though saving a high quality "master" file at this point is a good idea in case Step 2 (below) has to be repeated.
Second, after scanning to acquire what is reasonably all of the data in the original, you should downsample to the appropriate pixel size. Don't be concerned with PPI at this point. In fact, it is best to avoid saving any PPI value in the final file (e.g. by using Photoshop's "Save for Web & Devices..." option). Use a high quality tool to do the resizing and ony after resizing should you convert to the desired format; JPEG, GIF, PNG.
PPI in a digital image doesn't really exist other than as a note in the file's header that tells those applications that have their own concept of a virtual inch how the creator of the image wants the pixels to be scaled. A 300x300 pixel image tagged at 100ppi has exactly the same resolution (detail, sharpness, ...) as a 300x300 pixel image that is tagged at 300ppi. The only difference is that some applications will deflaut the image sizing on import to the spec'd PPI resulting in the first image being scaled to 3"x3" and the second to 1"x1".