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I have seen recommendations to scan analogue photo prints at a DPI setting anywhere between 200 DPI and 1200 DPI depending on the quality of the print. I've heard that everything from camera focus to print storage can blur the image, so the best way to choose is to scan at several different resolutions and compare them to see if the higher resolution captures any more detail. I'm wondering if anyone has seen any software to do this for you. I'm thinking you could just scan everything at 1200 dpi, then the software would down-scale as much as possible without losing "too much" information.

Side notes: I've seen recommendations for scanning negatives and slides go as high as 4k DPI. The same algorithm would probably apply, but I'm only concerned with scanning prints at the moment.

for the sake of argument, let's assume I'm going to reprint all these images at 10x magnification (e.g., for a mural). As such, I want the highest resolution image possible from the print, but for nearly any modern digitally printed image I will be simply wasting disk space if I scan at more than 300dpi since any detail smaller than that will just be noise or paper texture. I'm looking for a piece of software to analyse the image and detect an optimal resolution based on the characteristics of each image.

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Too large is better than too small, you can always resample it smaller. But no, the Best way is to know how to simply scan for the goal that you want to achieve.

Scan resolution just determines enlargement of the copy. But you can easily create far more pixels than you can use, and there's not much point of that.

The purpose of high resolution is for enlargement. It is not so much for more information in some nebulous manner, but specifically is to have sufficient information for the goal you want to achieve. So an important way to see it is this way:

There are generally two different goals for scanning an image, those goals being to show it on the video screen, or to print it on paper.

Print Resolution:

The ratio of (scanning resolution / printing resolution) is the enlargement factor.

For example,

Scan at 600 dpi, print at 300 dpi, for 600/300 = 2X size (double size or 200% size)

Scan at 300 dpi, print at 300 dpi, for 300/300 = 1X size (original size or 100% size)

Scan at 150 dpi, print at 300 dpi, for 150/300 = 1/2X size (half size or 50% size)

If you are going to print original size, just always scan photos at 300 dpi.

Antique contact prints might have as much as 600 dpi of detail available, but our printers to print it are more like only 300 dpi. Common color photos don't have much more than 300 dpi of detail. There is little point of scanning a Walmart color print at 1200 dpi.

Film is typically small, and so needs more enlargement, and thus higher scan resolution for film.

One example is to scan 35mm film at 2700 dpi, and print at 300 dpi, for 2700/300 = 9X size enlargement. 9X is about 8x12 inches (about A4 size) from full frame 35 mm (about 0.9 x 1.4 inches).
The ratio of (scanning resolution / printing resolution) is the enlargement factor. You don't need and cannot use more than you need. Plug in your own numbers.

This is called "scaling", and this enlargement concept is true for scanning anything, photo prints, documents, film, etc.

Video Screen Resolution:

The role of scan resolution for the video screen is to create the appropriate image size in pixels. Images are dimensioned in pixels. For one example, scan 6x4 inches at 100 dpi, which will create (6 inches x 100 dpi) x (4 inches x 100 dpi) = 600x400 pixels

Plug in the appropriate numbers to get the size you want from the size photo that you are scanning.

Your video screen size might be set to show say 1366x768 pixels (this varies, there are different size screens), and the concept is that this 600x400 pixel image will fill 600x400 pixels of that 1366x768 pixel screen (this example image width and height is near half of this full screen size dimensions).

That formula is true for printing goals too, meaning that if you will print 8x10 inches at 300 dpi, then of course in preparation, you need to create in the ballpark of (8 inches x 300 dpi) x (10 inches x 300 dpi) = 2400x3000 pixels

But again, (scan resolution / printing resolution) is the enlargement factor.

Example: If scanning 3.5 inches to print enlarged at 9 inches (at 300 dpi), then scan at 9/3.5 = 2.57 enlargement factor, or 2.57 x 300 dpi = 771 dpi. Your scanner offers a set of common (favored) resolution numbers (like 100, 150, 300, 600 1200 dpi). I suggest using the next larger number that your scanner offers (as opposed to entering 771 dpi). It's not rocket science, I might use 600 dpi for 771 dpi, close enough. But scanners may do slightly better using their one of their set of numbers (matches the hardware better).

One issue is that prints don't enlarge very well. More than 2x is suspect quality. Film is designed to be enlarged, and does much better.

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    I think you're missing a key third purpose — to create a digital archival copy. – mattdm Dec 5 '16 at 1:19
  • I don't see archive mentioned, but that is an old question, about how to plan in the absence of any known goal. Big as technically possible? Is that likely to be actually useful or needed? We should establish a goal, a purpose, hopefully reasonably usable, and then we know how to plan goals. If we just want to archive prints and possibly print copies, that is 300 dpi. Maybe dream big at 600 dpi? But color prints do not enlarge well, and such enlargement plans should be tested first, for usefulness. – WayneF Dec 5 '16 at 2:24
  • i didn't mention the display format since i didn't have one in mind. i have updated my question to point out that i want the highest resolution image i can get without simply wasting disk space. – james turner Dec 5 '16 at 18:44
  • based on your answer it sounds like i should simply scan all my prints at 600 dpi and accept the fact that i will be wasting 75% of my disk space on nearly every image. maybe some day i will find the software to compress the images down to 300 dpi. – james turner Dec 5 '16 at 18:46
  • Depends on your goal. If you imagine printing them double size someday, 600 dpi would do it (you may have already done that though). Or wanting a scanned 6x4 inch print to be 3600x2400 pixels for some possible future video screen size, 600 dpi would do it. 600 dpi should be bearable, not huge, and any photo editor will resample it. But you really ought to have some reason for any size you scan. The goal should be an image size, for some specific use - and Not just some arbitrary dpi number. Which dpi is like asking "how long is a string". The only answer is "what will you do with it?". – WayneF Dec 5 '16 at 21:09
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Nice question.

Forget about the fixed ppi. Forget about 200 or 300 ppi.

You need to think where all the image started.

Imagine you have a print that you know it came from a 35 mm negative. Well, let us scan first the negative.

Depending on your source, you might think a 24Mx image is simmilar of a 35 mm negative. See this questions (I wil not go in deepth on this):

Is it true that '80s 35mm photofilm had quality corresponding to 24 megapixels?

What is the equivalent resolution of a 35mm film

(Some people estimates that the resolution could be as low as 4-16Mpx or as high as 40Mpx. Depends on the film, ISO, development and quality of the initial shoot)

24Mpx it is a round number. On a 3:2 proportion is 6000x4000 px. Note the longer side. 6000px.

To squish 6000 px on 36mm (1.41") we need 6000/1.41 = 4255 ppi. Ok 4000 ppi is ok.

But I think much information is lost on enlargment a printed image. This needs to be studied further but you could limit the total megapixels of the resulting image. for example to 4500x3000 px. This is arround a 13.5 Mpx image, which is still a good size image (10-12Mpx still are a good size image).

Target Megapixels, 24Mpx - 13.5Mpx

If you have a 6 inches wide photo, that came from 35 mm film, if you scann it at 4500 ppi it would have your desired 13.5 Megapixels.

An 8x10 image could be for example scanned at 450ppi.

This is optimized for non textured paper, for example glossy paper and as photographic print.

If your copies are inkjet, a loooooot of information is destroyed. In that case you could scann them at lower resolution or resample them after, for example at half size.

If your photo came from a diferent sized negative, you need to do some aditional math, but probably starting with a 4000ppi base scan for film.

  • i don't have the negatives, and i believe the prints are limited by the system that created them. modern inkjet prints only have around 300 ppi of information. older prints generated with direct optical exposure can have more, but i've heard they top out around 600ppi depending on the system used to make them. it's interesting to note that a 8x10 might limit the max ppi further. – james turner Dec 6 '16 at 17:15
  • 1) It is ok if you do not have the negatives. The point is that you will not have more information than the negative. So any aditional resolution it is not worth it. 2) A modern inkjet print, makes a mess with the information. You can not rely on the 300 ppi it has no direct corelation. – Rafael Dec 6 '16 at 17:52
  • Target the total Megapixels ;o) and put a limit there. – Rafael Dec 6 '16 at 17:54
  • yes, i get there is a limit imposed by the 35mm film. but there is also a limit imposed by the print and a limit imposed by the camera focus. the point is that the limit of any individual print is difficult to predict and it would be nice to have software to do it for me rather than manually compare various resolutions. – james turner Dec 6 '16 at 18:40

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