Convert to CMYK Profile in Photoshop producing different results on Windows and Mac

[ukzembla]

Hi there, I've tried using "convert to profile" on an untagged RGB image from Shutterstock or similar into PSO Coated v3, and noticed that the PC produced noticeably different results (much worse / oversaturated) to the Mac. At first I thought that was down to the screen being rubbish, but the CMYK histograms are really noticeably different too – surely they should be identical if they're using the exact same profile for conversion, right? The ICC profile used is, unless I've made some stupid mistake, the same date, and exactly the same number of bytes. Has anyone else seen this behaviour? Is it something to do with the RGB profile of the screens, and Photoshop is trying to copy what the (rubbish) screen is displaying into CMYK? I'm only led to that idea because the converted and unconverted images look identical on both PC and Mac screens (as they should, more or less), but I'd expected the same inputs to have the same histogram output, right?

See attached (Mac histogram + image on the left, PC on the right). 

[Stephen Marsh]

@ukzembla 

There has to be a source, as well as a destination (in addition there are conversion options such as rendering intents, BPC and CMM).

It sounds like the assumed (working RGB) source profile is different on both systems.

[D Fosse · Accepted solution]

As Stephen says, a conversion implies a source as well as a destination. But you have an undefined source.

You need to start by defining the source, and you do that by assigning an RGB profile to the original. You just have to pick the one that looks right, try sRGB, Adobe RGB, Image P3. Then you can convert to PSO Coated, and then they will be identical.

If you receive untagged files you need to assign a profile immediately! Don't wait.

[Stephen Marsh]

As Stephen says, a conversion implies a source as well as a destination. But you have an undefined source.

Agreed! I call this an assumed source, as that is what working RGB is for an untagged/undefined file.

You need to start by defining the source, and you do that by assigning an RGB profile to the original. You just have to pick the one that looks right, try sRGB, Adobe RGB, Image P3. 

If you receive untagged files you need to assign a profile immediately! Don't wait.

By D Fosse

I facetiously call this "RGB ICC Profile Roulette", however, it isn't just random or luck, one evaluates the image visually and or with the info panel providing an informed basis for the decision to select a profile. 

https://prepression.blogspot.com/2014/06/rgb-icc-profile-roulette.html

[NB, colourmanagement]

Your screen colourpace is not used in the conversion to CMYK

Stephen and D Fosse are right, receiving an untagged image means that the system has to make an assumption, maybe your Win and Mac machines are set up differently (Photoshop Color Settings) the default RGB working colour space there is used as a source for the conversion (and viewing of) untagged images. 

By the way, why the hell are the library selling untagged images! It's resulting in Russian roulette.

You'll need to open the image on a calibrated screen and "try" assigning different RGB profiles until the image "looks " right. See what we mean about roulette! Then save with the profile embedded and try it on both machines.

I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management