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.

[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?

[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?

[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

[Henry Ambrose]

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

About that quote from your Scitex booklet -
(for friendly conversation and not just picking a nit)

I don't read this as verifying the internal corrections being made in 16 bit. What I see is that the AD (analog to digital) convertor outputs 14 bit which is then compressed to 8 bit for output. So the corrections may be made on 14 bit info but we don't know that for sure. They may just as well be made during the 14 to 8 bit conversion or after. I don't know. It'd be interesting to know for sure how it works. But I read nothing that indicates the info is ever 16 bit.

And about the appearance of histograms - Just because they are smooth doesn't mean they are good. A bumpy histogram may simply indicate that the file contains "bumpy" values which came from "bumpy" film densities which match the scene information.

[sanking]

About that quote from your Scitex booklet -
(for friendly conversation and not just picking a nit)

I don't read this as verifying the internal corrections being made in 16 bit. What I see is that the AD (analog to digital) convertor outputs 14 bit which is then compressed to 8 bit for output. So the corrections may be made on 14 bit info but we don't know that for sure. They may just as well be made during the 14 to 8 bit conversion or after. I don't know. It'd be interesting to know for sure how it works. But I read nothing that indicates the info is ever 16 bit.

And about the appearance of histograms - Just because they are smooth doesn't mean they are good. A bumpy histogram may simply indicate that the file contains "bumpy" values which came from "bumpy" film densities which match the scene information.

You are right. I am not sure how this is done, but the Scitex booklet clearly indicates that the analog data is captured in 14 bit, then converted to 16 bit, and eventually compressed to 8 bit. And I was told by a former Scitex technician that image processing was done in 16 bit. What would be the point in capturing in 14 bit - 16 bit, and then compressing to 8 bit before image processing? That sounds like brain dead technology to me.

In any event, that is all pretty much irrelevant. Doing the capture and saving the way I described, with this scanner, gives me better histograms, and better tonal values on the print.

Sandy