InvalidError :
Most people these days simply take a photograph of what they want to 'scan' instead of using an actual scanner.
For archival work (which this probably is if you're *that* interested in scanning 35-50 year old photos), you want a scanner. Camera sensors use a Bayer filter - basically 50% of the pixels record only green, 25% record only blue, and 25% record only red. A computer algorithm then uses that low-res RGB data to "make up" the colors in the neighboring pixels. A scanner OTOH will record the actual R, G, and B values of each individual pixel.
https://en.wikipedia.org/wiki/Bayer_filter
You can get away with a camera sensor if the camera resolution is much higher than the resolution of what you're scanning (e.g. a 30 MP DSLR picture of a grainy film photo). But for accurate digitizing at full resolution, you want a scanner.
JeppeArne :
I'm trying to find a good quality photo flatbed scanner. I have something like 500 slides that are 35 – 50 years old. Most of them are mounted in frames with protective "glas", but they are dusty anyway
Do you know what kind of slide film they are? If they're not Kodachrome, look for a scanner with infrared cleaning (aka Digital ICE aka FARE). Most slide films and dyes are transparent to infrared. So these scanners scan in red, green, blue, and infrared. The infrared scan should be perfectly clear, but dust, dirt, scratches, and fingerprints show up in the infrared scan. The scanner then knows where all the physical blemishes are, discards the RGB data in those areas, and guesses what the correct RGB value should be using an algorithm similar to a Bayer filter algorithm.
This saves a ton of work. I'd estimate about 90% of my time scanning film was going through each scan afterwards at 100% or 200% in Photoshop and blotting out dust and dirt with the healing brush. A scanner with infrared cleaning basically does this step automatically.
https://en.wikipedia.org/wiki/Infrared_cleaning
https://en.wikipedia.org/wiki/Digital_ICE
Unfortunately, Kodachrome is mostly opaque to infrared, so this technique doesn't work as well or at all on Kodachrome slides. Some of the later scanners were able to (sort of) do it.
I guess it would be nice to have a scanner that takes more than 4 slides at a time, and whose scan time is tolerably low, but the price will, of course, tend to be higher then.
I wonder about the extent of technical advance in the field of scanners. Would a 10-15 years old scanner model be an acceptable choice, or would I miss out on important advances?
I'd set the cutoff at 7 years or newer. That's about when USB 3.0 came on the scene. With USB 2.0, each slide could take over a minute to scan. USB 3.0 is much more tolerable. This may not matter if you're only planning on 2400 DPI (approx 8 MP) scans. It makes a bigger difference for 4800 DPI (30MP) and 9600 DPI (120MP) scans.
This is also shortly after the transition from CCD lamps to LED lamps. CCD lamps change color with age, so any scanner 10+ years old will need to be recalibrated, if the lamp is even producing enough of the primary colors to still be usable. They also changed color and brightness slightly as they warmed up, resulting in a 10-15 second pause at the beginning of each scan. LED lamps are very stable in color, and produce the proper color and intensity of light the instant they're turned on.
At 10 years old, you're probably looking at a dedicated film scanner. I actually wouldn't recommend these. They did the job back in the day, but they relied on a front-surfaced mirror angled at 45 degrees to view the slide/negative inserted horizontally. This mirror gets coated with dust, oil, and outgassed chemicals over time. You can remove it and have it professionally cleaned, but why bother when you can just get a newer flatbed scanner?
The flatbed scanners in the last ~5-7 years have rivaled dedicated photo scanners in quality when scanning film. You have to get the right type of scanner - one which has a light in the cover (either built-in or as an option). You place the slides/negatives on the flatbed, and close the lighted cover. The cover lights up the film from behind, and the scanning mechanism scans it. There are flatbed scanners which fake it with a mirrored cover, but these result in more noise since the light has to pass through the film twice and thus loses a lot more intensity by the time it reaches the scanning sensor. That lower intensity translates into more noise, just like a digital photo taken at high ISO.
When the slides have been digtized, the scanner should preferably perform well as an OCR document scanner for the occasional document (OCR) scan job.
I haven't seen a scanner ship *without* OCR software. Worst case, you can probably beg/borrow/buy a copy from a small business which bought a multifunction printer/scanner, but never use the OCR software which came with it. There's free OCR software as well, but you have to understand that the difference between 99.97% accuracy and 99.99% accuracy is 3x as much work correcting the resulting text. The small extra hundredths or thousandths of a percent accuracy makes a big difference.
I hope to get a used scanner and not spend more than approx. 150 dollars.
I am sure you have some good suggestions for me. Thanks very much in advance.
The one I have (Canoscan 9000F) is 7 years old and only cost me about $300 new. So you shouldn't have any problems finding it at ~$150. Lit cover and infrared cleaning. It's only USB 2.0, but the quality has been very good.
Edit: You may have to remove the slides from the glass frames to scan them on a flatbed. There sensor elements are usually focused on the surface of the glass. Scanning something even 1 mm above the glass can result in a blurry image.
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