U of S | Mailing List Archive | alt-photo-process-l | Re: OT: 16 bit editing myth or reality?

Re: OT: 16 bit editing myth or reality?

Title: Re: OT: 16 bit editing myth or reality?
Earlier this year I bought a high end flatbed scanner ( Scitex/Creo/Kodak EverSmart Pro) but when I got it up and running discovered that even though the actual analog to digital conversion was done in 14 bit, the saved file was only 8 bit. I consulted with some experts on the Scan Hi-End forum and here is what they recommended for scanning B&W negatives.

1. Do as much corrective work as possible in the pre-scan take advantage of the fact that the actual scan is done in high bit.

2. Scan in RGB.

3. Once the file is in Photoshop, and before any further corrections are done, change the mode from 8 bit RGB to 16 bit RGB, then change to 16 bit Grayscale.

This method gives much less combing of the histogram than making the scan in 8 bit grayscale, and then converting to 16 bit grayscale.

It is obviously much better to capture or scan in 16 bit if possible, which should be the case with most modern quality cameras and scanners. However, there are some high end desktop and scanners available on the used market that do not save in high bit and you can improve the quality of the image a lot by the work flow described above.

Sandy King

At 7:01 PM -0500 12/16/07, Ender100@aol.com wrote:
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Hi Yves,

I think your inquiry is interesting-especially the cumulative effect that adjustment layers would have on file data.

In the mean time, I have no doubt that 16 bit is far superior-and even to take it a "bit" further, I would venture to say that 16 bit RGB is better than 16 bit grayscale when working on the same "grayscale" file-the numbers seem to work better.  Bruce Fraser confirmed that with me (before he passed away).  I had contacted him when I was writing my book about some anomalies I found working with 16 bit gray scale files.

Anyway, good luck with your inquiry, I'll look forward to hearing your conclusions-in the mean time I have no doubt that working with 16 bit capture files is far superior to 8 bit.

Try doing some sort of series that makes an adjustment followed by one that reverses it and see how much error accumulates.  BTW, it has been suggested that even rotating a file can lead to some sort of degradation of data.

Best Wishes,
Mark Nelson

Precision Digital Negatives - The System
PDNPrint Forum at Yahoo Groups
In a message dated 12/16/07 4:31:46 PM, gauvreau-yves@cgocable.ca writes:



You could be right but I (we) don't know if PS works with real number until the data needs to be save or whatever. If it's not the case and this would be surprising, then each editing step would effect the data and each additional editing steps would have a cumulative effect. I don't think what you propose below is correct even if PS works with real number because the starting point would be different.


Let's say I choose arbitraly a normalised [0..1] pixel level of 0.5, lets see what our starting value could be in 8 bit resolution. This would mean we started with a value of 0.5*255=127.5 => 127 or 128, now in 16 bit this would be 0.5*65535=32767.5 => 32767 or 32768. Lets say our original pixel value is 32767, PS would transform this in 32767/65535 = 0.49999237 and we could proceed with editing from this value but if we convert this value to 8 bit and save it for later comparison, PS would need to convert this to a value to an integer value between [0..255], so lets do this by 0.49999237 * 255 = 127.4980545 and to 127 by rounding. If we need to apply the same editing to the 8 bit data file, PS will convert 127 to 127/255 = 0.498039215, a difference of 0.001953155, though very small this difference could be increased by editing the image further. Just out of curiosity say we apply a small gamma correction of say 1/1.45 and see what effect it as.


0.49999237^(1/1.45) = 0.6199955

0.4980392^(1/1.45) = 0.6183242

for a difference of 0.001671312


The difference is now lower, this could be surprising and it's why a full analysis including both color errors and quantization errors migth produce interesting results. I made a search on Google to see if someone did this kind of analysis but as of now (4) I saw only perceptual comparisons and no number crunching but interestingly the few test I've seen seem to indicate the difference are imperceptible from 8 or 16 bit editing on printed image. Food for thought don't you think?





PS I don't recall any discussion of this topic in particular but I have a bad memory so don't take my word for it.

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