Scanning printed photos

I was looking around for advice on what settings are best for scanning printed photos and I was amazed by the number of answers floating around on the great interwebz which are misguided, or are technically correct but miss the point. So, here is my advice.

First of all, let us define the goal: For a home user to scan printed photos so as to retain as much as possible of the visual information contained in the print, within reasonable limits, and without wasting too much space.

The answer, in a nutshell:  Scan at 600dpi, save as 24-bit-color compressed PNG or compressed TIFF. Do not use any of the fancy options for noise reduction, color correction, contrast enhancement, etc that might be provided by your scanner. If you need to improve something, edit the scanned picture later using your favorite image processing software, but never touch the original scanned files: always work on a copy of the original.

If the nutshell is good enough for you, then off you go, and happy scanning.

If you are interested to know why, read on.

Please note that the way the goal was expressed, we do not have to take into consideration what we are planning to do with the pictures later. When you scan, scan for posterity. Scan so that you can later crop, rotate, retouch, print. etc. If the photos need to be communicated through a low-resolution medium such as the web, then you can later make scaled-down copies for that purpose. But at the time of scanning, the point is to not limit yourself on what you will be able to do later with the scanned photos.

The resolution of photo prints ranges between 150 and 300 dpi. When we scan at 600 dpi, we are doing twice the resolution of a photo print, (or better,) which is the theoretical maximum required so as to capture all the information that there is on the photo print. Every bit of it. Not an iota of information left out. Sure, some color fidelity will inevitably get lost, but that's a different ball game altogether. As far as resolution is concerned, anything above 600 dpi is a pure waste of space.

24 bits per pixel is also perfectly adequate to fully match the capacity of film to convey color. There will be some reduction in color fidelity stemming from the fact that the sRGB color space into which you will most likely be scanning will not necessarily (and in all likelihood never) exactly match the color space of the photo, but the loss in fidelity will be imperceptible, and the measures required to correct this problem are disproportionately painstaking, and outside the "within reasonable limits" requirement stated as the goal in the beginning of this article. Furthermore, when you are working with differences in quality that are not perceptible, it is very easy to make a mistake which reduces, rather than enhances, the fidelity of the digitization, even by an imperceptible amount, and you will never know. So, color is best left at 24 bits per pixel sRGB and never messed with.

Use compressed PNG or TIFF because these file formats offer LOSSLESS compression. Lossless compression means that 100% of the information in the image is retained. Not very close to 100%, not imperceptibly different from 100% but precisely 100%.  Time and again I hear about people who save their pictures in uncompressed TIFF format; obviously, they do not understand squat about compression. It really is not rocket science: there are two kinds of compression, lossy and lossless. Lossy compression (JPEG) achieves huge savings, at a slight (usually imperceptible) expense to quality. Lossless compression (PNG, TIFF) achieves great, but not huge savings, at NO expense to quality. Using no compression serves no purpose, and is just plain stupid. It just spreads your picture over more sectors on your hard drive, increasing the chance that it will one day be lost due to a sector going bad.

Since lossy compression usually represents an imperceptible loss of quality, we could be scanning into JPEG, but the problem here is that if we ever retouch the picture, and then save it again as JPEG, the lossy compression will be re-applied, thus compounding the loss of quality. Keep repeating this cycle, and at some point the deterioration will start becoming perceptible. That's why we always use lossless compression on originals and on working copies, and lossy compression when publishing.

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