Scanning DPI

[Ben Syverson]

So I won't be buying a 4990 then. What's the SNR on a drum scanner?

It's not scanner-limited; grain limits SNR too. If you have grain that causes a 5 unit variation in a flat color area, it's the film itself which has a low SNR and which is bringing your effective bit rate below 6 bits / channel. So it will happen with any scanner.

Please god, will someone with a drum scanner please scan the same file twice, at 8 and 16 bit and run this workflow?

I know I'm going against the collective wisdom of the ages here, but the collective wisdom is wrong.

[percepts]

It's not scanner-limited; grain limits SNR too. If you have grain that causes a 5 unit variation in a flat color area, it's the film itself which has a low SNR and which is bringing your effective bit rate below 6 bits / channel. So it will happen with any scanner.

Please god, will someone with a drum scanner please scan the same file twice, at 8 and 16 bit and run this workflow?

I know I'm going against the collective wisdom of the ages here, but the collective wisdom is wrong.

So assuming you can get the SNR down very very low with a digital camera, then you can have much better colour fidelity with a digital camera than you ever can with film?

[Ben Syverson]

So assuming you can get the SNR down very very low with a digital camera, then you can have much better colour fidelity with a digital camera than you ever can with film?

I'm going to leave gamut out of the equation (that's a sensor / file format issue) and define color fidelity as the sum of the dynamic range of each channel. If that seems fair.

SNR is synonymous with "dynamic range." The greater the SNR, the more signal is above the noise floor, and thus the more levels of signal you can represent. In that case, for images with equal pixel dimensions, the image with greater SNR will have better color fidelity.

So in order to compare the digital camera with film, they must be the same pixel size. For example, my Canon 5D Mk II produces files with nearly the same pixel dimensions as my Nikon 35mm film scanner. In this case, the 5D wins, because after you factor in film grain and scanner noise, the digital camera has a far superior SNR, thus more DR, thus better color fidelity.

However, if you compare the 5D with an 8x10 image resized to the 5D's size (or vice versa), the 8x10 obviously wins in terms of color fidelity. While the SNR at 100% of the 2400 DPI scan is not great (maybe 30 dB if we're lucky), once we resize, the SNR shoots through the roof.

But in any case (35mm, 5D, 8x10) resizing for print will again alter the SNR. If we resize to a 300 DPI 8x10" print, the SNR of both the 5D and 8x10 (but not the 35mm, most likely) will jump beyond 48 dB (8 bit). Put another way, we are now in danger of banding. We can either do our color correction prior to print resizing, or we should convert to 16 bit.

So it's not such a simple question. SNR, being a relative measure, is only meaningful when comparing two signals of the same sampling frequency (ie, image size).

[Don Hutton]

Here's a tip (and you may find it crucial to your testing) - make the prints... then compare.

Clearly Bob has done it, I know I have, and I'll bet Sandy has. Why go to all this trouble is you're going to exclude the final product?

[Ben Syverson]

Contrary to popular opinion, nothing magical happens when you make a print. If there is no difference in the file, there is no difference in the print.

But if someone is willing to do prints, I'll send them two files and we can go from there.

[Ken Lee]

I just scanned a negative with lots of areas of smooth tonality, at 2400 spi on my Epson 4990. I scanned in both 16 and 8-bit B&W modes. I converted the 8-bit file to 16-bits, ending up with 3 files.

To each image, I added the same 18 Curve adjustment layers, in the same order - and then merged the layers.

In comparing the 3 images at 100% size, I am unable to distinguish them.

Of even greater surprise to me, I can see no banding or posterization in any of them. Even though the histograms of the images with lower bit-depth look more "spikey", the images themselves are, for all intents and purposes, identical.

Heavens to murgatroid.

(The negative by the way, is 4x5 Kodak TMY developed in Pyrocat HD. The lens was an old "junker", a barrel-mounted 190mm Bausch&Lomb Tessar which came with the 5x7 Kodak 2A I bought at an auction.)

[Ben Syverson]

In comparing the 3 images at 100% size, I am unable to distinguish them.

Crazy, isn't it?! Thanks for posting your results, Ken!

[percepts]

Of even greater surprise to me, I can see no banding or posterization in any of them. Even though the histograms of the images with lower bit-depth look more "spikey", the images themselves are, for all intents and purposes, identical.

Its the all the fault of histograms you know...

[Don Hutton]

Contrary to popular opinion, nothing magical happens when you make a print. If there is no difference in the file, there is no difference in the print.

But if someone is willing to do prints, I'll send them two files and we can go from there.

You've done prints? Because I have and I can tell straight up, that there are all sorts of issues which I cannot discern on my very nice and accurately calibrated monitors which do reveal themselves in the test prints I made when I did the same exercise from drum scans on a Howtek 4500 (which only has native 14 bit output which is interpolated up to 16 bit by software). I accept that the SNR on a drum scan could negate the result, except that I did the same test on my Epson 4990 about 4 years ago - and I did the whole exercise to print, because that it my final product... The different histograms Ken observed (the interpolated 8 to 16 bit histogram becomes spikey) actually can make become an issue... you may not see it on your monitor, but you may well see it pretty clearly when you make final prints. And of course, I found this out by doing a complete test from scan to print. Not sure what the point of a half test is...

[rdenney]

This is actually an exciting development. It means I can scan at 3200--somewhat beyond the capability of my V750--at 8 bit to control file size. Then, I can convert it to 16-bit and downsample it before making big adjustments. Those pixels won't be that accurate in and of themselves, for two reasons: 1.) the optics and mechanics of the V750 won't support it, and 2.) the samples will be small enough to see significant noise in the film itself, even Velvia. I have seen the effects of both of those reasons in my scans.

I have seen banding in my V750, when scanning in 48-bit color at 2400. Maybe I need to scan at a higher pixel density to eke a bit more grain detail, and then downsample that to average out the noise it will pick up. Scanning in 24-bit color would make that a lot easier.

I'm reminded of my old Minolta Multi II scanner that I owned before buying a Nikon 8000. That scanner had a resolution of only 1128 finished pixels per inch with medium format roll film, but it made beautifully smooth and accurate scans as long as you didn't need really big prints. I would often upsample the image as needed and it would still look good. It was smooth and accurate because each sample integrated out all the noise in the film. If we sample at the noise level in the film, then our sampling method only has to have a signal/noise ratio greater by about 3dB to be basically unnoticeable.

Time for some experiments.

Rick "thinking the distinction between scanning depth and working depth are critical" Denney