About wide vs. small gamuts, 8 vs. 16-bit &sRGB vs. Adobe'98

Started Apr 3, 2003 | Discussions thread
Donald Cooper Veteran Member • Posts: 4,927
Very relevant for 20D, needs a bump (nt)

Magne Nilsen wrote:

(This is an early preview of an article that soon will appear on
the http://www.etcetera.cc site)

With the many discussion appearing lately about colors, color
spaces, 8-bit vs. 16-bit and sRGB vs. Adobe RGB (1998), there seem
to be as many preferences as there are combinations, and most
proponents of one versus the other often seems more confused than
the other. I have a feeling that some perspective could be needed.
It is not too hard to understand most of this, so...

This is an honest attempt to set some basic terms straight. The
accompanying illustrations are made to perceptually convey what is
discussed, and are not meant to be scrutinized by their byte
values. They are for 'your eyes only'.

Let's simplify a little. The example above shows two gamuts. A
gamut is all the colors that can be reproduced or captured by a
device. The number of bits used to hold the gamut dictates the
number of steps that each color can contain. Many bits will give
room for many subtle variations or tones per color. The width of
the gamut puts constraints upon, or limits the saturation level of
colors inside the gamut. A wide gamut can hold very saturated
colors, while a small gamut will sacrifice those most vivid colors.
As long as the gamut is, and stays, both wide and has many bits to
represent the color values, few problems should occur in handling
images. As you see from the illustration above, the "large gamut"
version has more saturated blues, greens, reds, yellows etc. Why
would anyone consider the "small gamut" version at all?

These are the same gamuts as in the first image, but this time
represented with fewer bits. The large gamut version still has more
saturated colors, but now there is a price to pay! The "few-bits
large gamut" has sacrificed many of the more subtle tonalities to
give room for the more saturated and neon-like colors. Which finite
set of the colors above do you think would be the best to
accurately describe human skin, like in a portrait? Or - a sunset?
Or - a small lake with a green forest around?
These illustrations are much worse than the difference between
8-bit vs. 16-bit, and sRGB vs. Adobe RGB (1998), but this is still
the dilemma you are going to face in the real world of printers and
output devices. Sooner or later.
At some stage the "many-bits large gamut" image must be color-wise
transformed into a "not-so-wide" and "not-so-many-bits" gamut,
representing a display, a web page or a printer. Printer gamuts are
actually mostly smaller than sRGB.
At some stage you will be doing things in 8-bit software as well.
Photoshop has some limitations on the available functionality for
16-bit images, but I would still advice all to switch to 16-bit as
soon as possible, and stay there as long as possible when editing
images. When you go from “wide gamut 16-bit” to “small gamut 8-bit”
you should know what will happen, or you could be in for a
surprise, or even worse, your customer/client/uncle could be in for
a color-wise surprise.

As an example on how bad things can get, look at the last
illustration.

Applying an S-curve as strong as the one above onto the prior shown
“few-bits large gamut” image will result in a reduction from 400
finite colors down to 300. 100 colors (25% of the gamut!) will be
gone, and many colors will show up as identical in the image. The
same thing would happen if we were to show a “few-bits huge gamut”
image on your “not so huge gamut” display. You would not be able to
discern many of the colors from each other at all.

My intention with this is not to scare anyone away from using a
wide gamut working space to hold or edit their images. My advice
would be to check up what exact colors you are getting in Adobe RGB
1998 that are not present in sRGB and vice versa. I think you could
be surprised.

16-bit editing is almost without an exception an advantage.
Especially if you are going to use a wider gamut like Adobe 1998
RGB (or maybe even wider?). You will have a much bigger latitude in
you editing operations before you start to experience ugly 8-bit
problems like banding of neighboring colors, or posterization
problems in the shadows or in skies. Switching to, and staying in
16-bit as long as possible, is almost always an advantage for your
images. I think 16-bit is much more important than using Adobe RGB
1998 as a working space, but I constantly find professionals that
have heard that Adobe RGB 1998 is so good, but they still do all
their editing in 8-bit, and wonder why they have banding and
posterization problems. Most of them believe that this would be
even worse if they had sRGB as their working space, while the
opposite is true. I hope that became clear with the prior
illustrations.

Ah well, 16-bit is “better” than 8-bit then, but wide gamut is not
“better” than small gamut, although the words wide and small for
many would indicate so. It really depends. On the destination
target, on the software available, on your skills and knowledge, on
your input device/monitor/output devices level of quality,
calibration and exactness, and on a few physical truths like
bandwidth, memory, processor and hard disk size and quality. In
many cases it is actually an advantage to be working in a
“not-so-wide” gamut like sRGB. You get more subtle transitions at
the cost of some “neons”, and you will be much less surprised or
disappointed when transferring your images to the web, the printer,
the lab, the clients or your friends.

~~~~
Magne

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