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Old Jul 5, 2006, 7:54 AM   #1
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Am planning to buy a flat bed scanner to convert a lifetime's slides and prints. Don't quite understand why scans produce such large file sizes.

DSLRs produce good tiffs with file sizes around 10Mb to 20Mb and these are considered to be the equal of slides. However scanned slides result in file sizes in excess of 60Mb as far as I can tell. Why the greatly increased file size?

David

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Old Jul 16, 2006, 6:34 PM   #2
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David, this the closest answer to your question that I have in referrence material on hand. It doesn't really answer it as well as I'd like, but.....

This is from, A Few Scanning Tips, by Wayne Fulton:

"Scanners use a carriage motor, which takes about a minute to to sample one row of pixels at a time. Every pixel samples all three RBG colors. A 4000 ppi film scanner creates 4000x4000 = 16 million pixels per square inch of film, and creates an image size of about 5600x3600 pixels from 35mm film, or about 60 MB. It would take a 20 MP digital camera to create that, and there would still be a difference.

"Digital cameras sample all pixels at one instant, which means they are necessarily limited to sample only one color at each image pixel.

"...For output, the missing two colors at each pixel are filled in by interpolating from colors in the neighboring pixels."

So, I guess that maybe, a camera like the Canon EOS-1 Ds Mark II, at 16.7 MP is getting close to matching the resolution of film, but still falls short.

From what Wayne is saying about how digital cameras create a file, it looks like they are cheating by interpolating (mathematically guessing) 2/3 of the colors they output, while scanners sample each color for each pixel.

Since there is so much detail present in film negetives and slides, to pull this out, the scanner has to use high resolutions like 4000 ppi. That also gives you a lot of print size options.

A digital camera only records a 16 bit color depth in a TIFF image, too, and 8 bits for a JPEG. Most scanners nowadays have 48 bit color depth.

I wish this were a better answer, technically, but I don't have a lot of that kind of material.

If you're getting into serious scanning, check out Wayne Fulton's site:

http://www.scantips.com/

There's some good stuff, there.

Grant
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Old Jul 18, 2006, 9:08 AM   #3
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Thanks for your reply. I did have a good look at the website you referred to.

Would I be right to take a view that slides scanned at a lower colour depth of 16 bit would end up around as good as camera TIFFs?

If so, I could presumably scan less important slides and negatives at a lower resolution if I only expected to print them at smaller sizes, and scan best slides at the top resolution. Would this be a reasonable course of action?

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David
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Old Jul 18, 2006, 11:22 PM   #4
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David writes: Would I be right to take a view that slides scanned at a lower colour depth of 16 bit would end up around as good as camera TIFFs?

Hmm. Can newer scanners do this? Mine is five or six years old and there is no provision to adjust the bit-depth of a scan. Even if a scanner offers this kind of control, I don't think that it would be a good idea to scan film or slides at any less than maximum bit-depth. Higher bit-depth can equate with increased dynamic range capability, and you need a lot of dynamic range to pull image detail out of both the highlights and shadows.

As far as I know, most scanners don't output any more than 24 bits. Printers and video cards can't "see" any more than that. The extra bits aren't wasted, though. A scanner does a lot of processing of that 48 bit scan before it spits out the result to your monitor.

My version of Photoshop (7) doesn't support anything over 16 bits. I don't know whether the newer ones do, either. But, likely, when you save a scan to Tiff, you won't have any more than 16 bits anyway -- and 8 bits is usually good enough.

David continues: ...I could presumably scan less important slides and negatives at a lower resolution if I only expected to print them at smaller sizes, and scan best slides at the top resolution. Would this be a reasonable course of action?

Sounds reasonable to me. The only bugger is that if you found you wanted a bigger print of a lower-rez scan later, you'd either have to upsample to the new size -- which could, if the increase is significant, lower the quality, or rescan at a higher resolution. Depends on how picky you are.

Grant
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Old Jul 19, 2006, 12:53 AM   #5
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8 bits in Photoshop is 24 bits to a scanner. 16 bits in Photoshop is the same as 48 bits in a scanner. Photoshop can handle a 48 bit file from a scanner at full bit depth. Photoshop is 16 bits per color and the scanner is 48 bits per pixel. Works out to the same thing said different ways.

The best comparison I have seen of film and digital was a test where they kept the film completely in the chemical process and compared prints. They felt that good ASA 100 consumer color film was about equal to 6Mp and that Fuji Provia 100F was equal to about 11Mp. Provia is as good as it gets in 35mm color film. That pretty much reflects what a Kodak guru said in a technical paper, but he of course stopped at the ASA 100 Kodak Gold at 6Mp without mentioning Fuji.

Scanners aren't supposed to interpolate until you get above the optical resolution. So a 2400 X 4800 scanner would start interpolating above 2400 PPI. A printer with a 2400 X 4800 resolution will make prints that look a little better than a 2400 X 2400 printer. But a printer can actually print at 2400 X 4800 where a scanner can only scan at 2400 X 2400 or 4800 X 4800. My experience reflects most people's that a 4800 X 4800 scan doesn't look any better on a 2400 X 4800 scanner than a 2400 X 2400 scan. There isn't any reason to go higher than the lower number or actual optical resolution. You can do the same thing with an upsample in Photoshop.

It isn't that the film or slide scan has more information than a digital photo just because it is larger. And it isn't larger just because of bit depth. The problem is that there isn't a one to one relationship between film grain and pixels generated by the scan. The scan has to be at a higher resolution than a one to one would require to get all of the resolution available from the film. You can get increased resolution with a good film scanner to at least 4000 PPI, but there isn't really 4000 PPI of information on the film. You just have to scan that high to get all of the information that is available.

I started with Photoshop 3 and did my digital darkroom with a film scanner for years. I deluded myself into thinking that the file size and resolution of my film scans reflected a true advantage over digital. When I finally got a digital I was surprised that a 35Mb 16 bit TIFF from raw from my camera was actually better than a 35Mb scan from my film scanner as far as making a large print. My 2800 PPI film scanner didn't have enough resolution to compete with my digital camera.

Film scanners also accentuate grain. Flatbeds aren't quite as bad because the light source isn't as concentrated. But I was into Neat Image before I got my first digital camera. Noise reduction software works on film grain.

As far as scanning at lower resolution I always felt I wanted to do it only once and scanned at full resolution. I'm planning to get a good flatbed – probably a V700 or 9900F that will do 4 or 5 strips at once with Digital Ice and scan my entire film collection. I always just scanned film and slides I wanted to work with as it is a nuisance being able to do only one strip at a time. But I will scan them all at the highest resolution that continues to show improvement. I think the V700 goes over 6000 PPI optical and I might not see improvement up to that point. But once the scan is set up it will be at the best quality I can get from the scanner.

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