Why Use ProPhoto RGB Color Space With a Standard Gamut Monitor?

Many Inkjet Printer / Paper combinations can get close to the ProPhoto gamut, Monitors can not. So color adjustments must be done on-screen relative to an actual calibrated printer using that specific printer profile in PS will not hurt but will also be subject to the monitor's gamut clipping.  So the proper workflow is to Calibrate the printer,  then use that profile in PS to convert to from ProPhoto RGB.  Color correct from the print, not the display.  That way you will have a successful color-managed workflow.  

Tom, I was wondering precisely the same thing. You can really understand things thoroughtly and explain it to a six year old. Or you can understand it like a six year old, repeat it, and hope it is right. I am the latter. Here is my guess: A color-managed application like Lightroom that always uses the ProPhoto color space (wide) for working,  displays in a natural-looking way as possible on a real monitor (AdobeRGB or sRGB) by mapping the colors onto the monitor's profile (calibrated or default profile), with knowledge of the range of colors it is mapping from and the range of colors it is mapping to, allowing for a proper mapping. The accuracy of editing the photos will depend on the color gamut of that monitor, and you might be editing colors that you can't see.  If your printer has a wider gamut than your monitor (most do) then you might not get "what you see is what you get" if you print directly from the raw preview.  On the other hand, if you export into a viewable color space, say with a jpeg (into sRGB or AdobeRGB, depending on your monitor), and print the resulting export while viewing on a calibrated monitor, then you are more likely to get "what you see is what you get."

I recently found myself looking for a new monitor and found this thread after the same question came to my mind. I had a slightly different slant on the question, but I believe it's the same principle Tom was asking about. After some thought I think I've arrived at the answer so I'll post it here in case either:

a) it will be helpful to anyone else wandering this way, or

b) I can be told I'm talking rubbish.

Here goes...

From what I can gather, if an image has an Adobe RGB color space, and is viewed on an sRGB device, the colours will be under saturated.

I wondered (and I presume Tom does, too) that if you are in your photo editor of choice, editing an Adobe RGB image on an sRGB monitor, whether the same principle would hold.

This obviously can't be the case, as if you made the colours look right on your monitor and saved it that way, then it would effectively save an over-saturated image as far as the Adobe RGB color space is concerned.

This would also essentially be like trying to paint the Mona Lisa through a veil where you can't see the real colours that are being painted.

For this not to be the case, there must be some real-time conversion going on (either in the editing software, the OS (drivers?) or the hardware itself). There must be some interpolation-on-the-fly that converts between the image color space and the hardware one if they are different at the point of editing.

At least that's how I think it works. Can someone confirm?

There is confusion because the question in your Original Post is phrased very imprecisely,

Are you referring to using the Prophoto RGB color space inside Photoshop as your Working Color Space?  (Which is fine.)

The reason I'm asking for clarification from you is because the thoughtful answers by John Danek in reply #1 seem to be predicated on an interpretation of your question to mean you're talking about using PrpPhoto RGB as your monitor profile, and that would be massively wrong!  Equally wrong would be to use sRGB or Adobe RGB as a monitor profile. 

ProPhoto RGB, Adobe RGB and sRGB are standard color spaces, and they are defined as DEVICE-INDEPENDENT color profiles.  They should absolutely never, ever be used as monitor profiles.

A monitor profile must always be a DEVICE-DEPENDENT color profile, specifically describing your monitor color response characteristic.  It should be the profile you obtain after you calibrate your monitor, preferably with a hardware calibrator puck like the Eye-One (i1), Datacolor Spyder or X-Rite ColorMunki, then save that profile and set that saved file as your Monitor Profile in Photoshop.

Of course it takes an experienced and knowledgeable user to work with ProPhoto RGB as a color work space despite the fact that your monitor cannot show you all the colors.  That's where Photoshop's Gamut Warning (in the View menu of Photoshop comes in.  You need to know how to use the Soft Proofing capabilities of Photoshop.  Even though your eyes cannot see all the colors on the monitor, they can see the gamut warnings (definable in preferences, gray by default) on the monitor, and your printer can certainly print more colors than the monitor can show you, even wide-gamut monitors.

Of course this is relevant to printed output.  If you work exclusively for the web or for screens, then you can use the lowest common denominator, the narrowest color space of all, sRGB.  For some or many of us, only the photographic print matters and in that case only the ProPhoto RGB must be used.  Otherwise, you're discarding a lot of color quality in your printed images.

In sum:  Yes, of course there are advantages and benefits in working in ProPhoto RGB, with the caveat that you must know exactly what you're doing.

As the mantra goes, "real men only use ProPhoto RGB".

So, please clarify exactly what you meant to ask.  Thank you.

It's called rendering. Look it up.

Those color profiles are system based ad can be foud in Photoshop.  But, I have never used any of those profiles for any of my moitors.  The color space is RGB.  You can build an .icc profile for your monitor when you calibrate it.  Let me address each of your questions...

1., 1a.)  As stated above, I am not aware of a way of spplying a ProPhotoRGB to a monitor.  That goes for sRGB and Adobe RGB.  How many colors you can see depends on your graphics card that is driving the monitor.  Some plain jane monitors only display a few thousand colors where others can display millions.

2.)  Yes.  And they are obvious.  That's why you calibrate the monitor to work within your applications.  Even then, there are some colors viewed on the monitor that simply cannot be reproduced.  Even on the best systems out there.  The key is to control your display color through calibration and, during the calibration process, you store and apply a calibrated profile to your monitor, you can even ame it whatever you want and select it to be used by your display.  In this process, you typically see colors that align with your workflow, whether that be photographic or graphic arts.

3.)  You are confused.  There are calibration applications that can give you very realistic expectations when viewing a file and then printing that file.  A good workflow gives you the best of both worlds.  But, like I explained in #2, some colors on screen cannot be printed. You are talking about two totally different color spaces, one transmissive ( monitor ) and one reflective ( print ).  I personally do not like the term "softproof" or "softproofing".  The day I depend on my monitor to proof a job no matter what it is is the day I succomb to mediocrity ( sorry for the spelling, it's late ).

I hope you get an idea of what I am driving at.  Let me know if you have any other interesting questions.