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scan resolution and relationship to screen and print appearance

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I'm planning a large scanning project, and I'm trying to wrap my brain about the relationship between scan resolution and how the images will appear when printed or viewed on a large and/or high resolution screen. I'm pretty sure I'm missing some things (many?) in my understanding of how these interrelate.

Scanning Prints
If I understand correctly, printed photos (those printed years ago on photographic paper) have a resolution of about 300dpi. So, from what I've read, a 300dpi scan of a 4" x 6" original photo will look approximately (ignoring for the moment the inevitable degradation from making a copy in the first place) like the original if printed at 300dpi. If I want to print 4" x 6" from the scan, it should turn out reasonably well. If I want to print a larger image, I'm going to lose detail, because I'll be upscaling. So far so good?

If viewed on 4K screen with pixel doubling (so the screen shows 1920x1080), a 4" x 6" photo scanned at 300dpi will be 1200 pixels x 1800 pixels, which is pretty close to the 4K resolution, so the 300dpi scan should look decent on a 4K monitor or TV. Yes?

Scanning at, say 600dpi, doesn't get me any more detail at the sizes above, because the original source was created at 300dpi, and I can't create detail that wasn't in the original.

BUT, scanning at 600dpi does let me print a larger size (say, 8" x 10") without upscaling. And, if I had an 8K monitor (I don't), then the 600dpi scan would look as good on the 8K monitor as a 300dpi scan does on a 4K monitor. Yes?

Scanning slides or negatives
A 35mm slide or negative measures 36mm x 24mm (or 1.4" x .94"). So, a 35mm negative scanned at 3200dpi turns into ~4480pixels x 3000pixels. If I print an image from that scan, I'll be printing at 300dpi, which suggests I can make a good print in a size up to 15" along the longer dimension (4480/300 = 14.9). Now I'm beginning to wonder if I'm missing something, because somehow this doesn't quite pass the smell test for me.

Using the same logic applied to displaying on screens, a 35mm negative scanned at 3200dpi should look fine on a 4K monitor. In fact, it should look fine on an 8K monitor if the monitor is using pixel doubling. Is this really true?

Is my understanding anywhere close to right, or am I misunderstanding something about how all this works?
 
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I do not think that you are misunderstanding the numbers. The advantage of a higher resolution scan of a print gives you the option to down sample which could provide you with a slightly better scan if you have decent hardware, software and post processing skills. And yes, if you wanted to print larger than the original, you would not need to upscale, but that does not mean that the larger print is going to look identical to a reprint at the original size.

Slides and negatives have a lot of data, and should be scanned at a resolution that can extract their detail and meet your needs. Again, the quality of a scan will depend on hardware, software and post processing skills, so resolution alone is not a scanning cure, but it will hopefully address the theory of GIGO (Garbage In, Garbage Out).

What exactly is not passing "the smell test" for you?

--Ken
 
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I've taken a different approach for my 'film' scanning driven by two aspects of scanning:
  • Quality scanning can take a long time
  • I am not going to want a high quality scan of everything but will not know which I until I do; chicken and egg issue.
So, with these points in mind, I'm creating 'thumbnails' of all my film images which reduces scanning time but gives me a good enough rendering and documentation that I can go back and make a proper scan.

FWIW, here are my notes that I made for myself since this is a winter project and I knew I would not remember everything from one year to the next. You will see notes in there relating to a command line utility EXIFTOOL that I use for initial metadata branding. I assume you could do the same on import into LR.
 

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I look upon scanning as trying to replicate the pixels in a typical digital camera sensor. Assuming the 4X6 print was made from a file camera with a full size 35mm film image, the film frame was 24mmX36mm (the difference between the 24mm side and the 35mm film being the area needed for the film sprocket holes). The same 24mmX36mm size of the slide if you are scanning slides. So if today that same shot were captured with a modern full frame digital camera then for a 36mp digital camera. I would want to scan a slide at 7,360 x 4,912 px from my scan or converting that to inches (for the imperialists) that would need to be ~5200 ppi on the short side. The closest your scan could probably do is 4800ppi. For a 4"X6" print, 1200 ppi would get you that same 4800 pixels on the 4" side. Those would be my target scan settings. I would in reality opt for the highest pixel density that the scanner can handle because I can always resample the high resolution image to get something more manageable if my computer hard ware could not handle the highest resolutions.
 
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It is worth recognizing that while a print may have been produced a file at 300 dpi, it's a physical document, meaning that the ink will have spread across the paper and into the colors next to it, creating a wide tonality, which is a semblance of what we see with our eyes, which has nothing to do with resolution or dots.

The result is that scanning a digital print is not that different from scanning an analogue print, with its disorganized array of grain, or a negative or slide, with the same disorganized grain.

Along with these concerns is the viewing distance of a print. A self-critical obsessive, like myself, for instance, sees things others do not, and that includes other self-critical obsessives. For instance, I was at an art show with a fellow photographer, and we came upon a beautiful and striking print that was about 8' x 8'. Very detailed, high-frequency print. We were both blown away; however, I quickly saw evidence of chromatic aberration, while my friend did not; and even once it was pointed out to him, he did not care. Did this make the print any less formidable? It did not.

If the photo works, it works, whether on a screen or as a print. For instance, I was asked by a dear friend if I had a photo of her late son, an old friend of mine, that I could print for her. I found a 35 mm color slide of him among a group of six other people at a table in a dark restaurant, in a lively pose, smiling, talking to the person next him. I cropped everyone else out, turned the image to black-and-white, used a bit of noise reduction to reduce the grain, and made an 8 x 10 print that turned out so well I made a copy for myself and it hangs on the wall opposite my desk. It's a bit grainy, but content trumps everything.

It's worth noting that many of the most famous and well-studied photos ever made, going back well over a hundred years, are pretty grainy, including, over the last ten or fifteen years, images of rock stars taken in the 1960s that have become incredibly valuable in the form of very large prints made from high-speed negative film.

My routine is to scan prints at 600 ppi to 1200 ppi, and negatives or slides as high as my Nikon scanner will go, which is around 4000 pixels or so. 50 MPG camera-scans producing RAW images arguably make even better scans. Having all that resolution really enables me to do a lot of work on scanned prints without degrading them, and, in fact, sometimes making them better - and bigger - than the original print. As for negatives and slides, the technology to reproduce those gets better every year, so having high resolution scans helps future-proof whatever I or anyone else might want to do with them later.
 
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I look upon scanning as trying to replicate the pixels in a typical digital camera sensor. Assuming the 4X6 print was made from a file camera with a full size 35mm film image, the film frame was 24mmX36mm (the difference between the 24mm side and the 35mm film being the area needed for the film sprocket holes). The same 24mmX36mm size of the slide if you are scanning slides. So if today that same shot were captured with a modern full frame digital camera then for a 36mp digital camera. I would want to scan a slide at 7,360 x 4,912 px from my scan or converting that to inches (for the imperialists) that would need to be ~5200 ppi on the short side. The closest your scan could probably do is 4800ppi. For a 4"X6" print, 1200 ppi would get you that same 4800 pixels on the 4" side. Those would be my target scan settings. I would in reality opt for the highest pixel density that the scanner can handle because I can always resample the high resolution image to get something more manageable if my computer hard ware could not handle the highest resolutions.
I am also a big believer in never shorting yourself in resolution, but the counter to that is time to scan. Some scanners dramatically ramp up in the amount of time needed as resolution is increased, and I have not heard from many folks about the need to exceed 1200ppi. So, when one is having to look at a scanning project, I would think that time, and to a lesser degree space, might be factored in to the decision process as well.

--Ken
 
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Resolution

Scanner resolution has nothing to do with dpi. The true value is Ppi.

In todays digital workflow you only have dpi (Dot Per Inch) in ink-jet printers.

Even print houses uses Lpi/Lpc (Lines Per Inch or Lines Per Centimeter) calculated from the original image file from the Photographer, Delivered I Ppi (Pixels Per Inch).

It does not matter if he deliver in 72, 240, 300 or 1200 Ppi. If he just deliver 3500 Pixel on longest side for a print in best offset or ink-jet quality in 8x10” or A-4.

Film scannners, scan with a pixel line array measuring t.ec. 3500 Px. This “Optical” resolution Often marked underlined when choosing “Resolution” number in scanner attached software.

The 3500 Px are the scanners true resolution. If you chose other resolutions it is just interpolated in the same way as in Adobe Photoshop in Image – Image Size. Sadly in many old scanner programs you still see Dpi.

Some scanner “multiscan”, witch only gives a larger file size but no seriusly better detail of the film grain since the lens resolution quality are the same.

The scanner software settings does NOTHING with you scanner. You does not adjust the light in the scanner itself. It always scan in Auto – and then corrects in its software. So why not just crop and then scan in Auto and use a good software for further adjustment.

Scanning a 24x36mm film using a slow film scanner is today made better using to rephoto with a good camera and a fine macro optic (remember emolution towards camera) with a pixel chip measuring minimum of 4500 Pixels on longest side. If you want a larger file you can cut the rephoto in four and stitch.

The beautiful thing with Lightroom, both Classic and LR is that you don't see resolution in the Crop Tool only Pixel Size.

https://feedback.photoshop.com/photoshop_family/topics/are-resolution-outdated

https://www.tmax100.com/photo/pdf/film.pdf
 
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@Kierphoto:

Thanks for your detailed explanation. What you say makes sense (that pixel dimensions, not dpi, is what matters). And, now that I think about it, what I was doing to assess how a scan would look on a screen was to compute the pixel dimensions from the scan dpi and the size of the object scanned.
 
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