It's all about Gamut
Gamut is the word we use to describe a range of colours.
Of the standard RGB colour spaces, sRGB has the smallest gamut, Adobe RGB has a somewhat larger gamut, and ProPhoto RGB has a ridiculously enormous gamut.
High-end monitor manufacturers like to reference the Adobe RGB gamut when boasting about their flagship products. If you visit the Eizo or NEC websites, you'll see claims of "92% of Adobe RGB", "98% of Adobe RGB", etc.
What we never hear about is comparisons of normal monitors to the sRGB gamut. And you might be surprised.
To my understanding, the sRGB colour space was created last century to represent the average gamut of the imaging devices of the day, including CRT monitors. (Remember them?)
Like many people, I assumed that modern LCD monitors, with their amazing Contrast Ratios etc, must have far superior gamuts to those old dinosaurs. Turns out I was wrong!
The truth is, many LCD monitors have gamuts that don't even come close to sRGB. And laptops are among the worst.
Check your monitor's gamut
There's a quick way to see how your monitor's gamut compares to sRGB. (Please note this is not a precise scientific test, just a rough visual guide).
First, right-click and save this file, then open it in Photoshop.
These eighteen strips represent each of the six spectral colours fading to white, black and grey.
Open the Custom Proof Setup dialog:
"Device to Simulate" is your monitor profile (mine is called "6500_2.2_100_Eizo"). Turn off "Preserve RGB Numbers".
These settings are from CS2. The dialog differs in other versions, but it should translate ok.
Now turn on the Gamut Warning:
Wherever you see grey patches, that's the parts of sRGB that your monitor can't show properly. Here's my laptop test:
Wow! My laptop falls badly short of the full sRGB gamut.
And for comparison, here's my mid-range Eizo monitor:
Interesting, huh? Even a monitor which cost me more than my first car doesn't fully encompass the sRGB gamut. Notice how blues and magentas are the worst.
How does your monitor compare?
What does this mean?
In short, it means that if any of these extreme colours occur in your image, your monitor doesn't have the gamut to display them accurately. That's the key word here - "accurately".
Let's not get too uptight
Let's face it, the above test is hardly comprehensive. There's no skintones, or trees, or bridesmaids' dresses, or family pets. All we've tested are the really extreme colours, which (thankfully) don't occur in nature very often. Your day-to-day photographs won't present a problem at all.
But if you do photograph some very vivid colours, or if you get a bit heavy-handed with Saturation in Photoshop, you might find that you can't see detail in the brightest areas.
It's been fairly well documented that Photoshop's Gamut Warning feature is not particularly accurate. Those grey areas can only be considered a rough guide at best. But that's all we need.
Is this problem the same as Channel Clipping?
Only vaguely. Channel clipping is where one of the channels has reached the 255 value, so detail in that channel is lost. That problem can be faced by anybody, regardless of their monitor. (more info)
A small-gamut monitor presents a different problem. It can make your channels appear clipped even when they're not.
Ok, so how does Photoshop display colours?
Let's take a look behind the scenes ...
Colour-managed programs show your colours using the Relative Colorimetric rendering intent. I won't bore you with stuff about rendering intents (Google will give you plenty of info), but basically, it means that all colours that are within the screen's gamut are shown accurately, but any that are out-of-gamut all get flattened off to the nearest available in-gamut colour.
Gobbledegook? Let's try a diagram:
If the triangle is the gamut of our monitor, and we're viewing a photo with some very bright greens, what will we see?
Well, we'll see Colour A accurately, and we'll see Colour B accurately (although only just!), because the monitor is capable of them. But Colours C and D will look the same as Colour B, because out-of-gamut colours simply get mapped to the nearest available in-gamut colour.
So, when you look at the photo on your screen, all you'll see is a flat area of Colour B, but somebody with a better screen will be able to see the different shades of B, C and D.
Or, try this explanation:
On the left, we have a graph of reds in a photo - from no red at all to very bright red.
On the right, we have those reds displayed on a small-gamut monitor. The brightest reds are too bright to display, so they get "chopped off".
How do non-colour-managed viewers show colour?
Before you calibrated, you could see all the detail in a super-saturated image. Even after you've calibrated, you can look at your image in Internet Explorer, or Windows Picture Viewer, or some other "dumb" program, and see all the detail.
This makes you want to yell "Aaaaaaaaargh!!!", and throw your calibrator in the bin.
Why can simple programs show what Photoshop can't?
Well, it's simple. They do this:
They squash down all the colours to fit the screen's gamut. All the colours are now visible, but they're all wrong. Not just the bright colours - all the colours.
How wrong are they? Well, that depends on your screen. Some screens are pretty good, others are woeful.
The trade-off
So, here's your choice:
On the one hand, you can remain uncalibrated, and see all the brightest detail in your photos, but never know if your colours are correct.
On the other hand, you can calibrate, and enjoy correct colours most of the time, but work blindly when you have super-saturated colours.
Pretty crappy choice, huh? Sorry, but that's just how it is.
But, as I've been saying all along, it's not as big a problem as it seems. Most photos won't challenge your monitor's gamut in the slightest. So it won't surprise you when I say: "Calibrate, friend, and enjoy the accurate colours in 99% of your photos."
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Now, let's talk about those purple blues. Click here to continue.
If you have a question about this article, or any Photoshop question, please visit me at my "Ask Damien" Facebook page.
All content © Damien Symonds 2012