Converting 8-bit to 16-bit

[sanking]

16-bit files are much less prone to posterization than 8-bit files if one does a lot of tonal correction on the file in Photoshop. A while back I asked for suggestions on how to deal with some 8-bit drum scans of 12X20" negatives which I did not want to re-scan. I received some good adivice on this forum and have already applied it to several of the scans.

There is also another issue. Suppose you have a lot of existing 8-bit RGB files, or own a drum scanner or high-end flatbed that only scans in 8-bit. I asked this question on the ScanHi-End Group on Yahoo, and received a number of suggestions for converting 8-bit to 16-bit. The system that has worked best for me in scanning LF B&W negatives is this.

1. Scan in 8-bit RGB, doing as many tonal corrections as possible *before the scan." The reason for this is that many drum and high-end flatbed actually scan in 14-bit or 16-bit, and then convert to 8-bit for save. If you do as many corrections as possible before the scan these are done during the scan in either 14-bit or 16-bit.

2. After the scan, open the file in Photoshop and then before doing anything else, convert the file to 16-bit RGB. If you are working with a LF negative this is going to give you a huge file, so be patient.

3. Next, change the file from 16-bit RGB to 16-bit Grayscale, then save. You can do further corrections on this file as if it had been originally scanned in 16-bit.

If you look at the histogram after converting to 16-bit grayscale you will see that it is much smoother than an 8-bit grayscale file, with just a few spikes. Not as smooth as a file scanned originally in 16-bit, but not far off, and so far I have not seen any posterization with this method.

This method could also be extended to any existing 8-bit RGB files.

Sandy King

[vinny]

Thanks Sandy, i've been wondering about the difference in my large format scans myself. I made a scan from the imacon in 16bit and in 8bit. I could see a bit of difference but not much on my screen. I bet it would be more apparent on the print. Interesting discovery on the posterization.

vinny

[PViapiano]

There was info re this just a few months ago in the forum, in fact, everyone jumped all over a guy that was proposing 8 to 16 bit conversions.

The final word, though, was, once a file is in 8 bit, a conversion to 16 bit does not buy you anything. I understand what you're saying about 8 bit scanners actually doing their scans in 14 or 16 bit, and I can visualize that that could help you get a more robust file at scan time theoretically, but once it's knocked down to 8 bit, you cannot recover any of those lost bits.

I'm sure many more will chime in here...

[Jiri Vasina]

You don't recover any lost bits this way. But any further processing of 16-bit image should result in smoother transitions, there are more steps you can fill. So it's less likely you'll introduce any digital artefacts. If you don't do any curves/levels/burn-and dodging treatment or whatever, you don't gain a thing. But the more of these manipulations you do, the better to have the file in 16bit... (or at least that is the way I rationalize it)

[Ted Harris]

vinny,

The differences in a full range color print are subtle but immediately apparent. The Canon 5000 export driver processes images in 16 bit and sends them to the printer as 14 bit. I printed an image using the export driver and the "print w/preview" from PS CS2 and have now had dozens of folks look at the two prints. The 16 bit is the winner everytime.

[Ted Harris]

Sandy, that is interesting! Have you found any documentation taht verifies that your Eversmart Pro is actually working in the 16 bit mode when making prescan adjustments?

[Jack Flesher]

Thanks for the confirmation Sandy! I was one of the folks (or the one) who said there was benefit to converting from 8 to 16 before doing any processing as it gives you more adjustment range, and yes, I got jumped on.

But the fact is I had tested it out for myself long ago and confirmed that I got a better final histo by doing the conversion. And regardless of the naysayers, it does make logical sense -- again, you are not creating any new original tones, but are allowing finer gradations during any image manipulations after the conversion, which then helps resist banding in even-toned areas like skies as one main example.

Cheers,

[sanking]

Sandy, that is interesting! Have you found any documentation taht verifies that your Eversmart Pro is actually working in the 16 bit mode when making prescan adjustments?

Ted,

What I understand from reading the Scitex literature is that after the prescan adjustments the actual scan is made in 14-bit, which is then processed to 16-bit, and then compressed to 8 bit.

Quote from a Scitex EverSmart/EverSmart Pro "Product Booklet".

"The Scitex EverSmart scanner works with one Tri-linear CCDE, 8000 pixels with a built-in Anti-Blloming feature. The analog information is converted into 14 bit/color/pixel by A/D convertor and then compressed to 8 bit/color pixel."


Sandy King

[Larry H-L]

Hmm... I recently received some 8-bit Tango drum scans from 5x7 negs. When I converted them to 16-bit, I immediately saw banding in the sky areas. Not sure if it was real or a screen issue, but the banding went away when I converted back to 8-bit. Any thoughts?

[Brian Ellis]

vinny,

The differences in a full range color print are subtle but immediately apparent. The Canon 5000 export driver processes images in 16 bit and sends them to the printer as 14 bit. I printed an image using the export driver and the "print w/preview" from PS CS2 and have now had dozens of folks look at the two prints. The 16 bit is the winner everytime.

Ted, sorry if this is a dumb question but I don't understand what you're saying here. Are you saying that with two otherwise identical prints, one printed from a file sent to the printer as a 16 bit file, and the other as an 8 bit file, the 16 bit file produces a noticeably better print? In other words, if I do everything in 16 bit up to the point of sending the file to the printer and at that point make a copy of the file and convert it to 8 bit, then send both files to the printer (i.e. one 8 bit file, one 16 bit file), there will be a noticeable difference in the two prints? Or are you just saying that there's an advantage to doing what Sandy is talking about and converting an 8 bit file to a 16 bit file before editing it?