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8-bit and 16-bit images

 
 
Conrad
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      06-20-2006
Hi,

I have a question about running Photoshop in 8-bit and 16-bit mode.
When handling RAW images - I use the 16-bit mode for any adjustments
(larger color gamut). When handling JPEG images from camera - that come
in as 8-bit images - does it do any good to change these to 16-bit mode
while adjusting them in Photoshop?

Best,

Conrad
Camp Sherman, Oregon

 
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Michael J. Astrauskas
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      06-20-2006
Conrad wrote:

> I have a question about running Photoshop in 8-bit and 16-bit mode.
> When handling RAW images - I use the 16-bit mode for any adjustments
> (larger color gamut). When handling JPEG images from camera - that come
> in as 8-bit images - does it do any good to change these to 16-bit mode
> while adjusting them in Photoshop?


Conrad,

As I understand it, it will in fact do good if you're doing multiple
adjustments to your JPEGs as there will be less precision lost to
rounding errors.

--
- Michael J. Astrauskas
 
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Bill Hilton
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      06-20-2006
>Conrad wrote:
>
> I have a question about running Photoshop in 8-bit and 16-bit mode.
> When handling RAW images - I use the 16-bit mode for any adjustments
> (larger color gamut).


16 bit mode does not offer a larger color gamut ...

> When handling JPEG images from camera - that come
> in as 8-bit images - does it do any good to change these to 16-bit mode
> while adjusting them in Photoshop?


No, the damage is already done when you converted to jpegs, so leave in
8 bit mode for further edits.

Bill

 
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Bart van der Wolf
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      06-21-2006

"Bill Hilton" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
SNIP
>> When handling JPEG images from camera - that come in as
>> 8-bit images - does it do any good to change these to 16-bit
>> mode while adjusting them in Photoshop?

>
> No, the damage is already done when you converted to jpegs,
> so leave in 8 bit mode for further edits.


A small nuance though. If one uses several postprocessing steps, one
also accumulates rounding errors. So if more than one simple
correction is applied, it may help to convert to 16-bit/channel mode
first, and finally convert to 8-b/ch again.

Bart

 
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Bill Hilton
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      06-21-2006

>Bart van der Wolf wrote:
> A small nuance though. If one uses several postprocessing steps, one
> also accumulates rounding errors. So if more than one simple
> correction is applied, it may help to convert to 16-bit/channel mode
> first, and finally convert to 8-b/ch again.


Yeah that's the theory ... the reality is that no one can actually
provide an example of this where it matters ... go ahead, shoot a jpeg
of something, make a copy of it and convert to 16 bit, run the
identical steps on both the 8 and 16 bit versions and show us a
meaningful difference between the two final images.

So far as I know many have tried this experiment and all have failed to
show an example with a noticeable difference.

Bill

 
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Raphael Bustin
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      06-22-2006
On 21 Jun 2006 16:02:57 -0700, "Bill Hilton" <(E-Mail Removed)>
wrote:

>
>>Bart van der Wolf wrote:
>> A small nuance though. If one uses several postprocessing steps, one
>> also accumulates rounding errors. So if more than one simple
>> correction is applied, it may help to convert to 16-bit/channel mode
>> first, and finally convert to 8-b/ch again.

>
>Yeah that's the theory ... the reality is that no one can actually
>provide an example of this where it matters ... go ahead, shoot a jpeg
>of something, make a copy of it and convert to 16 bit, run the
>identical steps on both the 8 and 16 bit versions and show us a
>meaningful difference between the two final images.
>
>So far as I know many have tried this experiment and all have failed to
>show an example with a noticeable difference.
>
>Bill



It is *really* interesting and IMO quite instructive to
experiment with bit-depths in Photoshop using the
Image->Adjustments->Posterize tool. It takes a
certain kind of image to look *bad* at reduced
bit-depths.

Bit-depth and rounding errors are most likely (in fact,
*only*) observed in areas of minimal detail and smooth
gradients over a narrow tonal range.

Alas this situation does happen in landscape photos
with skies, so the issue is real. Even in the absence
of outright posterization, the effect is to make the
sky (or clouds, etc.) look "grainy."


rafe b
www.terrapinphoto.com
 
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Bill Hilton
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      06-22-2006

>Raphael Bustin wrote:
>
> It is *really* interesting and IMO quite instructive to
> experiment with bit-depths in Photoshop using the
> Image->Adjustments->Posterize tool.


What number do you plug in to "levels" to get a certain bit depth? I
don't think it works like you are assuming ... for example, if you
enter '256' how many total bits of color depth do you think you are
viewing?

> Bit-depth and rounding errors are most likely (in fact,
> *only*) observed in areas of minimal detail and smooth
> gradients over a narrow tonal range.
>
> Alas this situation does happen in landscape photos
> with skies, so the issue is real. Even in the absence
> of outright posterization, the effect is to make the
> sky (or clouds, etc.) look "grainy."


So where is an example? Typically the rounding errors would cancel
each other out. If they didn't, by some stroke of bad luck, I'd think
you'd have to run many operations to get enough round-off to see even a
couple of digits difference, which is a fraction of a bit. In other
words if RGB 44/67/99 somehow got rounded to say 44/66/99 that would be
a difference of one in 64 million. You can't see a difference this
small on your monitor, on a web image or in a print. So who cares?

The entire 8 bit vs 16 bit argument is interesting enough, but to me 8
vs 8 converted to 16 is meaningless. For prints I work on the 12 bit
(digital) or 14 bit (film scans) files in 16 bit mode, and most
photographers and authors agree this workflow has occasional benefits,
but there are a couple of guys who disagree even with this, like Dan
Margulis. They've asked for examples of files edited in both 8 and 16
bit mode that show noticeable differences (files that started life as
high bit) and it's actually hard to provide examples of this, though a
few have surfaced.

The 8 bit tiff changed to 16 bit tiff for editing (sometimes with a bit
of Gaussian blur added to induce noise) argument was brought up many
years ago (say Photoshop 4 or 5 era) but every author I've ever read
(guys like McClelland, Fraser, Blatner, Haynes, Caponigro) felt this
was pointless, based on their testing. Basically you just don't see
enough of a round off error to be visible with actual photos.

Doing it with jpegs makes even less sense since you've already skinned
off part of the image's finer points when you did the jpeg compression.

At any rate, if you can provide an example of an 8 bit jpeg that looks
visibly worse than the same file converted to 16 bit with the same
edits done to it in Photoshop I'd like to see it. If you can do this
I'll admit I'm wrong but so far as I know no one has ever supplied such
an example.

Bill

 
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rafe b
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      06-22-2006

"Bill Hilton" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
>
>>Raphael Bustin wrote:
>>
>> It is *really* interesting and IMO quite instructive to
>> experiment with bit-depths in Photoshop using the
>> Image->Adjustments->Posterize tool.

>
> What number do you plug in to "levels" to get a certain bit depth? I
> don't think it works like you are assuming ... for example, if you
> enter '256' how many total bits of color depth do you think you are
> viewing?


Plug in 32 (for example) and that's 2^5 levels, or 5 bits.
Plug in 64, it's 6 bits. I presume that's per color channel.
A highly detailed image may look just fine with as few as
16 or 32 levels (ie., 4 bits or 5 bits per color channel.)

> At any rate, if you can provide an example of an 8 bit jpeg that looks
> visibly worse than the same file converted to 16 bit with the same
> edits done to it in Photoshop I'd like to see it. If you can do this
> I'll admit I'm wrong but so far as I know no one has ever supplied such
> an example.



Probably no single operation, but a succession of operations,
most certainly. Here's what I tried:

1. Using Levels, adjust gamma to 1.1
2. Using Levels, adjust gamma to 0.9
3. Repeat steps 1 and 2 about eight or ten times.

With this procedure, I see obvious degradation of an 8-bit
image, but no comparable degradation of the 16-bit image.

I am "not" claiming that this is good practice or that it
represents real-world working conditions.

Nor would I claim that a single conversion from 16 bits/chan
to 8 bits/chan would "ruin" any image. That's poppycock.


rafe b
www.terrapinphoto.com


 
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Bill Hilton
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      06-23-2006
>>>Raphael Bustin wrote:
>>>
> >> It is *really* interesting and IMO quite instructive to
>>> experiment with bit-depths in Photoshop using the
>>> Image->Adjustments->Posterize tool.


>> "Bill Hilton" <(E-Mail Removed)> wrote in message
>>
>> What number do you plug in to "levels" to get a certain bit depth?


>rafe b wrote:
>
> Plug in 32 (for example) and that's 2^5 levels, or 5 bits.
> Plug in 64, it's 6 bits. I presume that's per color channel.


I used to think it works that way but now I'm not sure ... plug in 255
and if you are right you should see no change to an 8 bit/channel
image, but I see a change. When I compare to "indexed mode" images
(where you can plug in an exact # of colors but are limited to 256
total or 8 bits for all three channels total) I don't see what I would
expect either.

If you entered "2" for levels I'd expect 8 colors if you are right (3
bits total), but according to the Help files for Posterize (CS version)
this is not so ... the Help file says "For example, choosing two tonal
levels in an RGB image gives six colors: two for red, two for green,
and two for blue."

So either the Help files are wrong (possible) or it's not doing what
I'd expect.

>>bill wrote
>> At any rate, if you can provide an example of an 8 bit jpeg that looks
>> visibly worse than the same file converted to 16 bit with the same
>> edits done to it in Photoshop I'd like to see it. If you can do this
>> I'll admit I'm wrong


>Rafe wrote
> Probably no single operation, but a succession of operations,
> most certainly. Here's what I tried:
>
> 1. Using Levels, adjust gamma to 1.1
> 2. Using Levels, adjust gamma to 0.9
> 3. Repeat steps 1 and 2 about eight or ten times.
>
> With this procedure, I see obvious degradation of an 8-bit
> image, but no comparable degradation of the 16-bit image.


OK, I ran a quick test with these steps and you are right, you see
posterizing on the 8 bit image but not the same image in 16 bit mode.
Here's the example I ran, starting with a tiff, converting to jpeg with
8 level quality (which would make it more likely to posterize but
that's a reasonable quality level with Photoshop's jpeg conversion),
duplicating the 8 bit version and changing to 16 bit mode and running
the same steps on both six times.
http://members.aol.com/bhilton665/tests/8vs16.jpg

So you are right ... there's a guy on the Photoshop NG (I think Mike
Russell) who has held 16 vs 8 bit challenges in the past, with a $50
prize for anyone providing an example showing 16 bit editing is better
.... next time he runs the challenge I'll point it out to you so you can
submit an example

> I am "not" claiming that this is good practice or that it
> represents real-world working conditions.


Yes, if you have to make many edits you would be better off working on
something other than a jpeg, but still you've made your point.

> Nor would I claim that a single conversion from 16 bits/chan
> to 8 bits/chan would "ruin" any image. That's poppycock.


I'm not sure what you mean here ... I've never seen anyone claim
converting from one mode to the other damages an image.

Bill

 
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Bill Hilton
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      06-23-2006
To correct two errors in my earlier post ...

>Bill Hilton wrote:
> What number do you plug in to "levels" to get a certain bit depth? I
> don't think it works like you are assuming ... for example, if you
> enter '256' how many total bits of color depth do you think you are
> viewing?


Should be '255', not 256 ... 256 is illegal ...

> In other
> words if RGB 44/67/99 somehow got rounded to say 44/66/99 that would be
> a difference of one in 64 million.


Should be "one in 16 million" ... one of those days ...

Bill

 
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