Dan Margulis Applied Color Theory - 8 Bit vs. 16 Bit discussion

From: Andrew Adams, INTERNET:admsfmlyof4@earthlink.net
Date: Sat, Oct 30, 1999, 1:40 PM

Everyones response to my RGB gamut question was much apprieciated. Now for more questions (which I will post one at a time).

The first is concerning the 8 bit per channel delema imposed by Photoshop. If my thinking cap is on straight, aren't there many scanners that can now scan at 32 bits (16 per RGB) or even more? Now if the scanner doesn't convert it to CMYK, but instead supplies the prepress operator with the RGB file, that means the RGB has 16 bits per channel.

If that is correct, does PS do something to change it to 8 bits per channel as you open it? Or does PS hold on to the 16 bits while it is an RGB file, then after the CMYK conversion we can only have 4 channels of 8 bits each?


From: Andrew Rodney, INTERNET:andrew@digitaldog.net
Date: Sat, Oct 30, 1999, 10:37 PM

> The first is concerning the 8 bit per channel delema imposed by Photoshop. If
> my thinking cap is on straight, aren't there many scanners that can now scan
> at 32 bits (16 per RGB) or even more?

Not a lot but certainly quite a few. Photoshop can easily deal with "high bit" files (more then 8 bits per color). Not all functions will operate (no Layers, no filters other then the ImageXperss Deep Bit filters). But you can all the tonal and color corrections you wish in high bit (Curves, Hue/Saturation, Levels, etc) plus clone, crop, rotate, and do mode changes.

> Now if the scanner doesn't convert it to CMYK, but instead supplies the
> prepress operator with the RGB file, that means the RGB has 16 bits per
> channel.

It could be 10, 12, 16 bits etc. IOW, it's MORE then 8 bits per color.

Photoshop considers any file with more then 8 bits pre color to be 16 bits even though it may not be. It wasn't necessary for Adobe to place all kinds of different values in the menu; 16 bit basically is telling you that you have more then 8 bits per color.

> If that is correct, does PS do something to change it to 8 bits per channel as
> you open it? Or does PS hold on to the 16 bits while it is an RGB file, then
> after the CMYK conversion we can only have 4 channels of 8 bits each?

High bit scanners come in basically two flavors; those that use high bits but only provide 8 bits per color after the scan and those that can save out the actual high bit file. In the latter case, Photoshop can open the files and will indicate in the Mode menu that you have a file that is 16 bits per color. You'll also notice the Layers and filters menu grayed out. When you want to output, or do more "creative" work, you must convert from 16 bits to 8 bits. You'll notice your file now taking up half as much space as well.


From: Rick Boden, INTERNET:blphoto@cadvision.com
Date: Sun, Oct 31, 1999, 12:17 AM

>High bit scanners come in basically two flavors; those that use high bits
>but only provide 8 bits per color after the scan and those that can save out
>the actual high bit file. In the latter case, Photoshop can open the files
>and will indicate in the Mode menu that you have a file that is 16 bits per
>color. You'll also notice the Layers and filters menu grayed out. When you
>want to output, or do more "creative" work, you must convert from 16 bits to
>8 bits. You'll notice your file now taking up half as much space as well.

I wonder if anyone here can give advice on scanning in 16 bits with a Leafscan 45. The scans I get in this mode are all very dark and in the histogram all the information is crowded in the shadows. My attempts at correction result in some pretty weird colour effects. Any suggestions other than to just leave it alone?


From: Dan Margulis, INTERNET:76270.1033@compuserve.com
Date: Sat, Oct 30, 1999, 11:14 PM

Most scanners sample more than eight bits of data per channel. Not all of them are capable of exporting a file with more than 8 bpc, though.

If you have a file with >8 bpc Photoshop will open it as a 16 bpc document (often called high-bit) and you can then horse around with it to a considerable extent, although many commands are disabled. You *can* convert it to CMYK if you like. Sooner or later you have to go into Image: Mode and change it back to 8 bpc (often called low-bit), since otherwise you can't print it.

This is a theoretically superior method, but in practice there is considerable question as to how valid the additional data is. I tried to test this before I wrote my last book, where a couple of pages of the results appear in print at a size large enough to draw conclusions. I scanned a set of 10 originals on a very expensive flatbed scanner, and a different set on a medium-priced desktop scanner that is capable of exporting a high-bit file. With my 20 high-bit files, I made 20 copies and converted them immediately to low-bit.

With the 20 high-bit files, I really tortured them with a series of curves much more drastic than would be used in real life. As I was doing this I was saving the changes as Actions. When I was done, I converted the high-bit files to low-bit, as I would have to do if I wanted to print them. Then I played back the same Actions on the files that were originally low-bit, and saved the results.

I knew that the ones that were worked on in high-bit originally would be better, but I didn't know how much. I was hoping to find out whether there were certain categories of image where it made sense to work on them in Photoshop in high-bit mode. High-bit files are twice as large as low-bit ones, so it's a PITA to use them.

The results surprised me. From the cheaper scanner the results were virtually identical. From the more expensive scanner the quality improvement was there, but it was so marginal that you wouldn't see the difference on output even on a film recorder. In print, maybe if you applied drastic curves *and* you found out later that you had to print the file at four times the size you originally thought, it might make a noticeable difference. Or maybe, as scanners get more accurate, the extra bits will start to make more of a difference.

So, from what I've seen so far, working in high-bit in Photoshop gets you a better-looking histogram, but not a better-looking image.


From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Sun, Oct 31, 1999, 2:06 AM

>The first is concerning the 8 bit per channel delema imposed by
>Photoshop.

Only for Lab files is there a limit to 8-bits per channel. You can use 16-bits per channel for either RGB or CMYK files. For RGB this means it would be a 48-bit file (3 x 16 = 48) or for CMYK is would be a 64-bit file (4 x 16 = 64).

> If my thinking cap is on straight, aren't there many scanners
>that can now scan at 32 bits (16 per RGB) or even more? Now if the
>scanner doesn't convert it to CMYK, but instead supplies the prepress
>operator with the RGB file, that means the RGB has 16 bits per channel.

A scan that's more than 8-bits per channel is referred to as "high bit". So a 30-bit scanner is using 10-bits per channel; a 36-bit scanner uses 12-bits per channel; a 42-bit scanner uses 14-bits per chanel. For each additional bit, you double the amount of grays PER CHANNEL. So you're getting HUGE amounts of additional data for each additional bit.

In any event, high bit scans use the 16-bit file format in Photoshop and other applications for that matter. There is 8-bit and 16-bit, nothing in between in terms of support. (This means if your scanner uses 14-bits per channel, you still end up with a 16-bit per channel image; it's just that the two extra bits aren't being used.)

>If that is correct, does PS do something to change it to 8 bits per
>channel as you open it?

No. You get a 16-bit per channel image in Photoshop with limited support for tools, and no support for filters (unless you get a plug-in that let's you use a few tools and filters in 16-bit mode).

> Or does PS hold on to the 16 bits while it is an
>RGB file, then after the CMYK conversion we can only have 4 channels of
>8 bits each?

If you have a 16-bit RGB file and convert to CMYK, you end up with 16-bits per channel there too; you just get an extra channel, just like when you're in 8-bit mode.


From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Sun, Oct 31, 1999, 1:38 AM

>So, from what I've seen so far, working in high-bit in Photoshop gets you a
>better-looking histogram, but not a better-looking image.

I agree, but I think it depends on the image. Tough images that are very dark or very light will definitely be helped by high bit scans because in 8-bit mode, detail that COULD be captured isn't captured because the surrounding area will report the same values or too high/low due to averaging.

In REALITY, I have seen LOTS of posterized grayscale images (on screen and in output) because they just didn't have enough data in them to begin with BEFORE even getting to levels and curves. I think editing exacerbates an already existing problem more than it does to create it (in a real world situation).

You can also MUCH more easily see posterization when working in larger color spaces, such as Lab, Wide Gamut RGB, ROMM RGB (to a lesser extent) and Ektaspace (also to a lesser extent); this is because the space is bigger, and gaps in the histogram become visible quickly due to the large "steps" from on value to the next; i.e. going from 128 to 129 Red is a bigger step in a large gamut space than it is in a smaller space.


From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Sun, Oct 31, 1999, 2:21 AM

>I wonder if anyone here can give advice on scanning in 16 bits with a
>Leafscan 45. The scans I get in this mode are all very dark and in the
>histogram all the information is crowded in the shadows. My attempts at
>correction result in some pretty weird colour effects. Any suggestions
>other than to just leave it alone?

Hrmm, I don't know if it will scan high bit; BUT if you have a gamma setting, you can set it higher and then the software will shift more bits to shadow areas and hopefully lighten up your images so you can better see shadow detail.

On an Agfa DuoScan T2500XL for example, I had to bump it up to a gamma of 2.2 for reflective and 2.5 for transmissive to get suitable detail out of the scans.


From: Andrew Rodney, INTERNET:andrew@digitaldog.net
Date: Sun, Oct 31, 1999, 7:59 AM

> I wonder if anyone here can give advice on scanning in 16 bits with a
> Leafscan 45. The scans I get in this mode are all very dark and in the
> histogram all the information is crowded in the shadows. My attempts at
> correction result in some pretty weird colour effects. Any suggestions
> other than to just leave it alone?

The Leaf doesn't "tone" it's high bit scans like many of the newer scanners (Imacon) that does provide high bit output. IOW, it's perfectly normal for you to see this effect from your Leaf. You simply have to open up the file in Photoshop and pull some radical curve or usually move the sliders in the Levels dialog. This seems like it would really toast the file but since you have "raw" scanner data, that's not so. In the case of scanners like the Imacon, they do this for you saving you time (usually with these kinds of scanners, you are dealing with pretty large files anyway and having them in high bit means they are now twice as big). The toning saves time and energy.

As to the color, that shouldn't be an issue, at least with the Leaf I had. After pulling the levels, the color remained as it looked in the Leaf interface. But Leaf hasn't updated their plug in for years and it's possible as the OS matures, it's starting to fall apart. Plus the Leaf plug in has NO idea about RGB Working Spaces so it's throwing raw (hopefully calibrated) RGB to the screen. In most cases, what you see in Photoshop AFTER the scan isn't what you saw in the Leaf preview. What's your RGB Working Space? You may need to pick a space that's as close to the condition of the raw monitor as possible so both previews match.


From: Rick Boden, INTERNET:blphoto@cadvision.com
Date: Sun, Oct 31, 1999, 12:16 PM

> The toning saves time and energy.
>As to the color, that shouldn't be an issue, at least with the Leaf I had.
>After pulling the levels, the color remained as it looked in the Leaf
>interface. But Leaf hasn't updated their plug in for years and it's possible
>as the OS matures, it's starting to fall apart. Plus the Leaf plug in has NO
>idea about RGB Working Spaces so it's throwing raw (hopefully calibrated)
>RGB to the screen. In most cases, what you see in Photoshop AFTER the scan
>isn't what you saw in the Leaf preview. What's your RGB Working Space? You
>may need to pick a space that's as close to the condition of the raw monitor
>as possible so both previews match.

Thanks for the reply. BTW, I'm the guy from the filmscanner list who cooled the CCD on the Leafscanner.

If you have been sucessfully working with 16 bit scans on the Leaf in the past then I am encouraged and will put some more effort into it. The working space I am using is ColorMatchRGB and our 8-bit scans look terrific. I will try again and maybe get back with some more questions.

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