Dan Margulis Applied Color Theory

RGB to CMYK Conversions

RGB to cmyk conversions
Posted by: Jeff Natrop
Sun Nov 12, 2006 12:40 am (PST)

Hello All,

A bit of background for my question:

I'm new to this forum. I'm a commercial advertising photographer, 26yrs exp, converted to full digital capture in 2000, proficient in PS, Have studied Photoshop 5 Color Correction by Dan Margulis.

We mostly supply RGB files along with an inkjet reference print that matches our Hardware calibrated crt monitors-LaCie. I'm proficient at color correction.

Although most of our digital files need very little if any color correction. We gray balance our shots when shooting. We color balance each of our studio lights and light modifiers. I color match to specific product colors when a tight match is required.

It's been my experience that the people who do the cmyk conversions of our rgb files do a poor job. Images print darker and flat. They all to often just hit the "convert" button. Printers don't use ICC profiles. I've had many tell me "it dosen't matter what profile I use, they always strip out any profile. Just give me the file in cmyk." I've asked "what flavor of cmyk-swop coated, uncoated, web, sheet fed?" They always say it doen't matter.

So I'm trying to gain info on the to cmyk conversion process. I want to provide better conversions for our clients. I have done a few and the results have been good, but I want more knowledge / experience before I sign my name to anything.

My question to you all is; what books would your recomend to study that will give me the info I need to make good cmyk conversions. An understanding of the use and settings in photoshop CS for UCR, GCR, dot gain, total ink limits, etc.

Dan's class would be great, but maybe a little overkill, but I would go in a heartbeat. But my boss would never spent the $$$$, and I don't have that kinda pocket change.

Most of the time, the clients, art directors, designers, don't know any info about the press, paper stock,press profile, cmyk profile. And I'm supprized how little the press people seem to know about RGB to CMYK conversion. Or about corrections/enhancements in cmyk. (Other wise our work would look much better in print).

So givin that info, what the best info you can give me about making cmyk conversions.

Thanks for the advice

Jeff Natrop
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Re: RGB to cmyk conversions
Posted by: "MARK SEGAL"
Sun Nov 12, 2006 8:35 am (PST)

There are two excellent books which devote substantial space to this subject:

<Real World Color Management> by Fraser, Murphy and Bunting (Peachpit Press), and
<Color Management for Photographers> by Andrew Rodney (Focal Press).

Both are available from Amazon.com at reasonable prices.

Mark Segal
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RGB to cmyk conversions
Posted by: "Andrew Rodney"
Sun Nov 12, 2006 8:35 am (PST)

On 11/11/06 8:33 PM, "jeff090756" wrote:

It's been my experience that the people who do the cmyk conversions of our rgb
files do a poor job. Images print darker and flat. They all to often just hit
the "convert" button. Printers don't use ICC profiles. I've had many tell me
"it dosen't matter what profile I use, they always strip out any profile. Just
give me the file in cmyk." I've asked "what flavor of cmyk-swop coated,
uncoated, web, sheet fed?" They always say it doen't matter.

Glad to see you recognize what a silly mindset this is and how ineffective such suggestions are by these print vendors. Of course it matters and of course you need to know the specific conditions, ideally described by an ICC profile. Once you have that recipe, an excellent conversion process is almost a few commands away.

My question to you all is; what books would your recomend to study
that will give me the info I need to make good cmyk conversions. An
understanding of the use and settings in photoshop CS for UCR, GCR,
dot gain, total ink limits, etc.

I feel you pain. This is one reason why I devote an entire chapter in my book to photographers working with CMYK and press issues. It covers the concepts surrounding UCR, GCR, TAC etc with language geared to the photographer along with some strategies for working with such clients. If you©ˆre interested in a seminar that deals with this and more in a photo centric way, look into the D65 seminars by Seth Resnick ( http://d-65.com/).

Andrew Rodney
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Re: RGB to cmyk conversions
Posted by: "Romano, John"
Sun Nov 12, 2006 8:35 am (PST)

Jeff

I would go with the USWebV2 profile that is in Photoshop, if you need to go to CMYK.

Printers dont use ICC Profiles ??????

That is an Assumption on your part.....FIND a new printer.

They dont care, its doesnt matter are KEY words that say they do not understand ColorManagement.

Again the best advice would be find a new printer....no matter how you supply your files they could and most likely will hose your files.

John R
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Re: RGB to cmyk conversions
Posted by: "Jim Rich"  
Sun Nov 12, 2006 9:49 am (PST)

Jeff,

It great that you are interested in learning about color separation. And there are a few resources about color separation that have or will be mentioned.

However I have an observation, question and comment about your post.

The observation is that you and other photographers and printers have the mindset of " us and them". This has been going on for years so it is nothing new.

And I know that these days pre-press and color printing is more of a commodity and is price driven. So if a printer wants your money and wants to keep your business it seems logical that if the printer makes you the client unhappy that they stand a chance of losing you as a client.

So how have you considered suggesting to the boss why don't we change that us and them mindset to we. And if the service provider does not get it, then move on to some other printer who is likes that idea of that as well as your money?

Just a thought.

Jim Rich
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Re: RGB to cmyk conversions
Posted by: "Ric Cohn"
Sun Nov 12, 2006 11:05 am (PST)

Good question, and I hope other's will have some CMYK suggestions. In the past when faced with a lack of information I've begged off doing the CMYK conversions on the assumption that my "guessed" settings will be no better on average than theirs. Personally, I'd concentrate on figuring out why the files you've supplied haven't looked good. Are you sure it's not your files? Is it because the printer is ignoring the RGB profile? What is your RGB working space? How are you profiling your monitors? How old are they? If you've built profiles for your printer to match your monitor have you had proofs made by an outside source to confirm you are truly running to a standard?

Unless I have had a discussion with the printer and know they will honor my RGB profile I supply the files in Lab. If they don't know what to do with a Lab file you can be pretty sure they're clueless about anything except their CMYK (if that) and they would have botched the RGB. If you really need to supply CMYK without knowing the conditions, Dan's book has a lot of information on CMYK.

Ric Cohn
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Re: RGB to cmyk conversions
Posted by: "John Denniston"
Sun Nov 12, 2006 3:43 pm (PST)

Hi Jeff,

The decision of what "flavor" to separate with should be a simple one unless you don't know what kind of paper and press will be used. If you don't know, and the art directors don't know, maybe the printer doesn't know and is waiting for the arrival of instructions closer to press time. He will adapt what ever you send him to his press conditions.

I am a great advocate of the photographer setting his own ink limits, dot gain etc during the conversion to CMYK but unless you have a lot of experience this requires a very close relationship with the people running the press so that feedback is not only honest but an on going process. It sounds like you are using a variety of printers so getting the necessary feedback is not going to be easy.

If your conversions to CMYK are better than what is done by your printers then you are doing something right. I would build on that. Analyze what you sent, the digital file and the proof, how you separated it, what it was printed on, what it looked like when printed and how it differed from the proof. Buy one of Dan's latest books to help you understand the process.

One suggestion I would make is that it sounds like your printers are using North American General Purpose Defaults to separate but have modified the defaults to ignore profiles. If you are sending them AdobeRGB files they are opening them in sRGB but ignoring the imbedded profile so the pictures appear washed out, flat and possibly a little dark on their monitor. So would the separations they make and, of course, the results when printed.

Regards,

John Denniston
www.dennistonphoto.com
www.dirtbikephoto.com
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Re: RGB to cmyk conversions
Posted by: "Matthew Rigdon"
Sun Nov 12, 2006 3:45 pm (PST)

Hey you considered buying some books for your boss, maybe something from Tom Peters? ;)

figure you need to set aside a couple of hundred dollars for books, RWCM, CM for photographers, Professional Photoshop, that still leaves you with a hundred bucks left for other training. I just joined lynda.com. It's just $25 a month (the cost of five mochas at Starbucks or a movie). I joined mainly for web design and flash training, but they do have a prepress video series up, about 8 hours, by Taz Tally. I haven't watched any of it, but maybe someone can comment on it who's familiar with the trainer.

I like lynda.com for two reasons: it's nice to hear a voice and watch someone do stuff, even if it is just a video. And two, they're geared more towards designers, not technical people. I'm going through their CSS tutorial right now and it's nice to see how to make logos appear, not hear about the technical reasons why inheritance works the way it does.

So figure you have to invest three to four hundred dollars on yourself in a year, it's a good investment. If it comes down to betting on you or betting on your boss, I wouldn't put my money on your boss. From your original post, it sounds like your employer has a lacksidasical attitude towards quality. Fortunately, your clients don't care, either. If you're in a fairly small market, maybe you guys can just keep going, but what happens if someone comes in and starts doing superior work for the same price? Is your company going to THEN make the changes? They haven't been preparing for it. If you've taken it upon yourself, you then have the choice to stay on or jump ship and work someplace better. Or go it alone.

If you're looking for a reason to spend your own money, I think your boss's attitude is your answer. If no one above you cares, and they're the ones driving the company, where do you think the company's going? It almost sounds like you could send the files off to Vistaprint and get the same results.

HTH,
Matthew Rigdon
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Re: RGB to cmyk conversions
Posted by: "Ron Kelly"  
Sun Nov 12, 2006 5:52 pm (PST)

On 11-Nov-06, at 8:33 PM, jeff090756 wrote:
Images print darker and flat.

Jeff:

I'd say that is what you get when you do conversions by "default".

I suggest this:

1.Get a few images which are typical of the work you do. Make CMYK conversions to SWOP v2, then open up and make your final corrections to the image. Put together a collage or use page layout software to show several images on one 8.5x11 page. Save as a flattened tiff with no profile.

2.Send to a few printers that you do/have done business with. Get a proof from them.

3. Make your own proofs using RIP software and your own choice of proofing printer. It doesn't
have to be big and expensive.

4. Compare your proofs with what you get from your printing companies. If necessary, make a 2nd or 3rd . . . (infinite?) iteration until you get to a point where you feel that you have some reasonable feel for what will happen. Hopefully this seems like an improvement from where you've been.

I've oversimplified things a bit perhaps but this is the only way that I can see to get "there" from "here".

BTW: Dan's books are your best reference. After you've had some experience in this cycle, re-read them.

Cheers,
Ron Kelly
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Re: RGB to cmyk conversions
Posted by: Jeff Natrop
Sun Nov 12, 2006 11:30 pm (PST)
 
Ron,
Thanks for the input. It all makes good sence to me.

Could you expand on RIP software. I've asked several folks and forums as to what RIP  software does and why do I need it / spend the $$$$ on it (it's a bit pricy). But I don't get  much of an explenation.

Do you have a recomendation?,
Why is it necessary?
Does price make a difference?

Jeff Natrop
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Re: RGB to cmyk conversions
Posted by: Jeff Natrop
Sun Nov 12, 2006 11:37 pm (PST)

Jim,

I don't know if I agree with your stament of "mind set of us and them" but I won't argue about that.

And I know that these days pre-press and color printing is more of a
commodity and is price driven. So if a printer wants your money and wants to
keep your business it seems logical that if the printer makes you the
client unhappy that they stand a chance of losing you as a client.

So how have you considered suggesting to the boss why don't we change that
us and them mindset to we. And if the service provider does not get it,
then move on to some other printer who is likes that idea of that as well as
your money?

I think you missed the part where I said:

"Most of the time, the clients, art directors, designers, don't know any info about the press, paper stock,press profile, cmyk profile."

What I ment was that as a photographer, my contact is with the art directiors, designers, product managers. All who usually don't know who the printer is going to be. That is deteermined further down the line. My access to press info is very little, if any. When I do get a contact at the printer, they have very little info to give out. Or are not willing to give it out. Sometimes its hard to tell if the know, but don't want to give it to me, or if they just don't know much.

So that's why I want to learn what I can to give them the best cmyk file I can. Taking into account how to set or use UGR,GCR, total ink limits, black ink limits, dot gain, etc.

Jeff Natrop
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Re: RGB to cmyk conversions
Posted by: Jeff Natrop
Sun Nov 12, 2006 11:42 pm (PST)

If your conversions to CMYK are better than what is done by your printers then
you are doing something right. I would build on that. Analyze what you sent,
the digital file and the proof, how you separated it, what it was printed on,
what it looked like when printed and how it differed from the proof. Buy one of
Dan's latest books to help you understand the process.

Hi John,

Your response shows that you understand my situation and post.

I'm looking to understand what or how to set / use UCR, GCR, dot gain, total ink limits, black ink limits, etc.

I don't work with printers. My connection as a photographer is with the art directiors, designers, product managers. They in turn may or may not deal with the printer. It may be further down the line. They (art directiors etc.) often don't know who is going to be the printer. That gets decided further down the line on the progect. In an ideal situation, all the needed press info, paper stock, etc. would be available. But I've found that just doesn't happen very often if at all. So I need to be able to do the best I can with what I can get. So that's why I'm looking to find that info and educate myself in the arena of rgb to cmyk conversions.

thanks
Jeff Natrop
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Re: RGB to cmyk conversions
Posted by: "Stephen Marsh"  
Mon Nov 13, 2006 3:06 am (PST)

Jeff, this is obviously a "new" situation for you, but not for this list. This is a common issue, you will find many similar threads in a search of the Yahoo list or you can read some consolidated threads at Dan's site:

http://www.ledet.com/margulis/ACT_postings/ACT.htm

I will attempt to make this short and concise, but I promise nothing!

If I understand your posts, this is your situation (it varies):
1) Sometimes you hand off RGB with a inkjet reference print. The RGB flavour may be of your choosing, or it may be part of a delivery spec.

2) Sometimes you hand off CMYK (often in despair when you see what happens to your RGB files). You obviously have the discretion to do so in these cases rather than hand off RGB.

2b) You also mentioned colour correcting some shots, which would seem to be a hit and miss affair if you are after any sort of close match to an expected hue or tone with your "partners" upstream being unknown and or dropping the ball. These could just be "general" corrections that work well anywhere and you are not after a close match.

This is how I would play things in your situation - presuming that you are working mainly for magazine or flatsheet type print in the ballpark of SWOP or more specifically TR001 type CMYK conditions:

1a) If handing off RGB files, I would continue to ICC tag them as you currently do - but I would be wary of using Adobe RGB or wider spaces to hand off to strangers for possible varied use upstream. I would use sRGB for want of a better standard (which used to be Apple RGB or ColorMatch RGB). There is no guarantee that they will *still* not hose your files by ignoring the tag, but perhaps you will luck out and match the assumed RGB space.

Some may even prefer to convert to CMYK first to set the gamut mapping/compression and then convert back to RGB and "fix" the shadow endpoint - rather than accept anothers "push button" conversion. But there is still no guarantee that the person doing the conversion will handle the RGB correctly even in this case.

1b) You mention a reference print. This means very little if printed from the RGB file with no CMYK simulation and is likely ignored by the publisher or prepress who will run their own soft/hard proofs as they know the final condition and trust their own proofing over others, even as reference.

Optional) As suggested earlier in the thread, LAB mode is another file delivery option that has it's own set of issues, both good and bad that get around people ignoring profiles (as Photoshop LAB is a standard that Photoshop can't ignore!).

Proofing RIP's were mentioned in the thread which are good for volume production users that have to manage both raster/vector in layouts. For a Photographer only printing images and not layouts, Photoshop may be all you need - as it provides all the tools one needs to make a press (colour) simulation on an inkjet. You need the right stock and inks of course.

I would include some sort of "label" at the foot of the print indicating that it is a "reference print" in SWOP CMYK or whatever, with your contact details and other required info on there too. Again this may be for nothing as far as the final end user is concerned, but it is better than just printing a ProPhoto or Adobe RGB image from Photoshop to the wide gamut inkjet profile (compared to press CMYK). There are good reasons to do a press simulation for your peace of mind and even more so, once the blame game comes around (but it is only useful as evidence if you can prove that your inkjet press simulation is accurate)!

2) When handing off CMYK files, get as much info as you can. If you can't get much info then don't worry too much - welcome to the less than ideal real world! Newsprint is not expected, so the baseline would be magazines in SWOP type settings. So settle on a good SWOP or TR001 profile to use, knowing that your expectations are more "open".

I commonly use custom CMYK, but that is not TR001 as is the v2 Adobe profile (but it is SWOP, which is not a colorimetric specification). I find th Adobe v2 TR001 profile too heavy in the black for most images/work. I can't be bothered channel mixing better GCR from this profiles conversion, so if I need different UCR/GCR or TAC in TR001 conditions, then I use the Chromix family of SWOP TR001 profiles. Quark also has a useable SWOP profile and there are many others out there (which may or may not be TR001 even if they are SWOP).

As it is rare to find a "family" of profiles for a common print condition, if one wishes to play with GCR or TAC one is pretty much stuck with legacy Custom CMYK or post separation edits (unless a profile can be made or found with the desired GCR, Chromix freely offered these).

I would not ICC tag the images, but I would use file names that include either SWOP and or TR001 (or whatever condition the aimpoint of the separation is). I would include the ICC profile with the images so that interested parties upstream can use it if required. You may choose to tag the files with a CMYK profile, that is your call.

It seems to me that GCR should not be a concern for you at this stage, you have more elementary things to sort out. That being said, if your K plates are similar to the SWOP v2 profile and you don't like the print results in magazines - and you can't put the blame on file handling or printing...then it is time to use a lighter K plate, despite the theory that say's more GCR is better (it can be, but perhaps not for every image).

Best,

Stephen Marsh.
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Re: RGB to cmyk conversions
Posted by: "Ron Kelly"
Mon Nov 13, 2006 9:40 am (PST)

Jeff:

RIP software is not strictly necessary. In theory, it just allows you to print with Postscript functionality (postscript fonts, for example) to a non-postscript printer such as most inkjets.

However, I use a RIP for CMYK work. Why?

The particular RIP I use is called PowerRIP X and I like it because of the calibration feature they call "dot gain curves".

Some RIPs, for example one from HP, have to ability to use profiles so that everything should match in theory. My commercial printer claims to meet SWOP V2 and when I invoke the corresponding profile to drive my inkjet via the HP RIP it should match their proof. It doesn't come close enough for me. With the HP RIP, well, that's all there is.

PowerRIP allows an additional curve to be used for output. I set up PowerRIP to use SWOP and the output looks the same as the HP RIP. However, by invoking my own "dot-gain curve" (I'm not sure that this is the proper use of the term but that's what PowerRIP calls it) I can dial in the final result to a very close match.Calibrationists will probably call this a hack and terrible practice, etc. but it works, and works well.

For example, a common problem is too much magenta in the blue skys. It's easy to see in a photo or two with different blues, but really hard to sort out in flat colours. With a variety of images you can soon tell if the magenta is too high or the cyan too low, etc.

One crucial point: as I mentioned before you must have a sample set of photographs to use this process on. Don't try and dial in your correction using a bunch of flat colours only.

I have a two-page set of images that I use for this purpose and it works well for me. It has also got a bunch of colour swatches and a couple of gray ramps.

I'm not suggesting that I would discount other RIPs, I think if I was doing it over I might try the ColorByte which I've read some good things about.

I think you're going to have to commit to the minimum investment to do good cmyk work. It will take a few dollars and some time, but if you and your company are committed you'll make it. Just don't give up.

Cheers,
Ron Kelly
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Re: RGB to cmyk conversions
Posted by: "Jim Rich"
Mon Nov 13, 2006 2:12 pm (PST)

Ron,

I hear ya and I understand what ever works.

But... these days, a good ICC based rip that takes advantage of linearization methods and good icc source and destination profiles will give you a reasonable visual match right out of the box.

What I did not say was that some printers can be unstable and so you somehow someway check if they have drifted.

If you buy into the measuring methods a linearization, you can keep your printer inline without having to get a head ache. You just let the measuring and lin process do the heavy lifting for you. The implication ( and common sense) is that a lower cost person can do the linearization and or the higher priced employee does not have to be tweaking the printer.

And of course all of this comes at a price. That is, what do you want to pay for tools such as a spectrophotometer vs how much do you want to waste in time and materials. Which ever one is comes out to the lowest price works for me.

Mileage will vary.

Jim Rich
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RGB to cmyk conversions
Posted by: "Andrew Rodney"
Mon Nov 13, 2006 2:16 pm (PST)

On 11/13/06 9:47 AM, "Ron Kelly" wrote:

RIP software is not strictly necessary. In theory, it just allows you to
print with Postscript functionality (postscript fonts, for example)
to a non-postscript printer such as most inkjets.

To be clear, we should point out that a RIP doesn't necessarily have to support Postscript (that's why you'd be paying more for a Postscript RIP). There's a big fat licensing fee going to Adobe. For example, the ImagePrint "RIP" you mention can be purchased with PostScript functionality or without at a lower cost (in fact, talking with the president at Photo Plus, he told me that they will likely stop offering Postscript as an option since so few of his customers need it and order it).

Without the Postscript engine, the product is still able to RIP (rasterize) some data or, if used with raster files, it's now really not a RIP but rather a print environment. I think of it as a substitute print engine.

The current print drivers for most photo printers don't support direct use of CMYK since Quickdraw and GDI drivers don't know how to handle it. If using CMYK is necessary for output to your ink jet, you should consider a "RIP" or substitute print driver. ImagePrint supplies profiles in RGB but will accept CMYK data for conversion back to RGB for the driver. The ColorBurst RIP accepts RGB and CMYK data as well but their paper profiles are CMYK. Neither is right or wrong but just the manufacturers take on the best way to profile their print stream.

The ColorBurst RIP (which can be bundled with the Pro line of Epson printers) is really ideal for doing CMYK proofing (it's so called SWOP certified which may or may not mean a darn thing depending on how you fell about certificating such products).

The particular RIP I use is called PowerRIP X and I like it because
of the calibration feature they call "dot gain curves".

Both RIPs above do allow for this kind of linearization which again is the prime reason to consider tossing the supplied print driver and using a 3rd party product. The ColorBurst RIP is very robust in building print "environments" using an EyeOne Pro or DTP-70 for custom linearization.

For example, a common problem is too much magenta in the blue skys.

That's an output profile issue, not necessarily a color space/driver issue. With custom profiles, my skies don't go magenta using either the Epson driver or a 3rd party RIP. Skies shift because the incorrect numbers are going to the output device. This was discussed in length when talking about the U.S. Web Coated (SWOP) v2 profile. When used for a device that really does conform to TR001, you get no such shifts. I have the output to prove it. If you send that data to a device that doesn't behave as the profile expects, shifts are going to show up.

Andrew Rodney
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Re: RGB to cmyk conversions
Posted by: "Matthew Rigdon"
Mon Nov 13, 2006 2:23 pm (PST)

It sounds like you're set within a normal production pipeline, so I don't understand why you need to give CMYK files to anybody. It's the Art Director's job to get that sort of thing taken care of. He should have a layout designer who's familiar with doing the separations to handle the RGB > CMYK conversion for you. Unless the printer is doing the layout as well, in which case someone should be a liason between you and the printer to make sure everyone's on the same page.

A general knowledge of how the process works will certainly benefit you, but the way you're plugged into this, it sounds like you need to just focus on taking shots and doing the RGB edits in Photoshop to fix color and retouch. Dan made mention that sRGB may be a good color space choice since it creates fewer out-of-gamut colors for CMYK later. I've heard some mention that Colormatch RGB was designed by prepress for doing RGB edits that end up in CMYK. I don't know if Colormatch is still considered a good choice for that.

Are the designers asking you to send your files directly to the printer? Don't you have to hand them off to the designer who then sends them over? Does this vary from job to job? It sounds like, in the system you're in, you should never have to do a CMYK conversion. There are other people in the chain who are supposed to be paid to do that sort of thing. Now, if you understand more about how the conversions are done, it can obviously help you do corrections in RGB that will hold up through the process.

I don't know if the printer is necessarily to blame. They really shouldn't be talking to a photographer directly. Your art director should be the go-to guy. But I know at the last company I worked for (lots of 2D and 3D design for web and CD-ROM), the art director just wasn't very good. He studied product design and all the artists absolutely despised him because he never knew how to do his job, not mention he didn't know how to do any of their jobs. It was sad because they hired him from outside, when they had a guy in the art team who was better qualified and should have been promoted. Instead, the guy who was passed up just left for a better company. If your art director isn't doing his job, that may be why the printer is acting the way they are.

Matthew Rigdon
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Re: RGB to cmyk conversions
Posted by: "Matthew Rigdon"  
Mon Nov 13, 2006 4:55 pm (PST)

Since I mentioned lynda.com in my email, I actually checked what they have in prepress. There's only one training set listed, a Photoshop CS for Prepress, the presenter is Taz Tally. It's 8 hours total and I joined lynda.com for other stuff, so I'm not going to go through the whole thing, but since I mentioned it here on the list, I decided to watch one of the videos. I picked the first RGB > CMYK movie.

One thing he said was stay in RGB as long as possible, which I agree with. then he talks about color spaces. His recommendation was Adobe RGB, but he says "some prepress houses use Colormatch RGB, which is a color space designed for prepress." Don't know where he got that from and I've never heard it before, but there you go.

I hope this doesn't violate any rules, since I'm only discussing three minutes out of an eight hour presentation, but I thought if anyone ever considered joining lynda.com it might be worth knowing a little about the prepress info there and how accurate it is.

It sounds as if he's wrong about how Colormatch was developed, but I don't know if Colormatch is a good RGB space for CMYK (or it's kind of an old wives tale of sorts).

Matthew Rigdon
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Re: RGB to cmyk conversions
Posted by: "Andrew Rodney"
Mon Nov 13, 2006 5:02 pm (PST)

On 11/13/06 2:19 PM, "Matthew Rigdon" wrote:

I've heard some mention that Colormatch RGB was designed by
prepress for doing RGB edits that end up in CMYK. I don't know if
Colormatch is still considered a good choice for that.

Not at all! ColorMatch RGB is a color space invented by Karl Lang, the designer of the PressView (and later Sony Artisan). The target calibration values produced on a PressView provided chromaticity values and a TRC gamma that was named by Karl as ColorMatch RGB. When all PressView displays where at their calibration aim points, that display produced ColorMatch RGB. Prepress had nothing to do with this.

Andrew Rodney
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Re: RGB to cmyk conversions
Posted by: "Michael Plack~Metro"
Mon Nov 13, 2006 9:34 pm (PST)

On the other hand, since ColorMatch, as well as sRGB, which has been mentioned in this thread as another alternative color space from which to convert to CMYK, are closer in size to CMYK, why wouldn't they be a safer bet than larger spaces like AdobeRGB for the CMYK conversion?

Michael Plack
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Re: RGB to cmyk conversions
Posted by: "Ric Cohn"  
Mon Nov 13, 2006 9:35 pm (PST)

On Nov 13, 2006, at 6:17 PM, Andrew Rodney wrote:

I've heard some mention that Colormatch RGB was designed by
prepress for doing RGB edits that end up in CMYK. I don't know if
Colormatch is still considered a good choice for that.

Not at all!

Andrew, was this answer referring to Matthew's first sentence, second sentence or both. I've not heard ColorMatch disparaged with a "Not at all!" before.

Ric Cohn
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Re: RGB to cmyk conversions
Posted by: "John Denniston"
Mon Nov 13, 2006 9:35 pm (PST)

HI Jeff,

You are in a worse position than I thought.

One thing you could do that would take time but not money is find a typical picture and separate it using as many possible combinations as you have time for. Save the pictures with a file name describing the separation method, including the dot gain etc. but without any profile attached. Make sure you have your colour prefs set to ignore CMYK profiles when opening files.  I assume you're printing to coated and uncoated stock so it would be best to make the first separation using North American defaults and then switch to custom CMYK and using the SWOP coated and uncoated starting points, separate the same picture using various combinations of dot gain, GCR and ink limits both total and for black. You could also do some of the separations from an sRGB work space rather than AdobeRGB to see how the appearance changes.

When you open the pictures each will appear to be different and you can compare the results. How they appear on your monitor has nothing to do with how they would print but if you compare the individual plates of each picture with another you will get a sense of how changes in ink limits etc change the appearance of the image.

Dan has a chapter in one of his books on ink limits, levels of GCR and dot gain and how they affect the final result.

Regards,

John Denniston
www.dennistonphoto.com
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RGB to cmyk conversions
Posted by: "Andrew Rodney"
Tue Nov 14, 2006 7:32 am (PST)

ColorMatch RGB was not designed for Prepress or CMYK in anyway since all it is, is the color space of a PressView when calibrated to it©ˆs target calibration. ColorMatch RGB is a display color space. It was (and still is) a working space before Adobe or anyone else had ever heard of the term working space. ColorMatch RGB is the color space of a calibrated display in a very fixed, well defined condition. I'll add, so is sRGB. Due to the phosphor set and other hardware issues, the gamut of ColorMatch RGB exceeds sRGB a bit. The TRC gamma is also different (1.8 versus 2.2).

Andrew Rodney
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Re: RGB to cmyk conversions
Posted by: "Ric Cohn"
Wed Nov 15, 2006 7:54 pm (PST)

On Nov 14, 2006, at 9:07 AM, Andrew Rodney wrote:

ColorMatch RGB was not designed for Prepress or CMYK in anyway since all it
is, is the color space of a PressView when calibrated to it’s target
calibration.

My understanding/memory may be bad, but I would disagree. My understanding/memory is that the PressView monitors were expensive professional monitors that date from a time when almost all work by retouchers was for CMYK output. The sales point of the PressView monitors was their superiority for working toward CMYK output. This all seems to me to say that the ColorMatch space was designed to aid in a prepress workflow. I believe the fact that we now recognize it as a Monitor space is as irrelevant as the fact that AdobeRGB was created completely by accident when someone at Adobe entered a wrong value and sent it out into the world.

Ric Cohn
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Re: RGB to cmyk conversions
Posted by: "Andrew Rodney"
Thu Nov 16, 2006 10:52 am (PST)

On 11/15/06 7:29 PM, "Ric Cohn" wrote:

My understanding/memory may be bad, but I would disagree. My
understanding/memory is that the PressView monitors were expensive
professional monitors that date from a time when almost all work by
retouchers was for CMYK output.

If you'd like I'll send you Karl's email privately and you can discuss this with him. As to the intent, yes at the time, MOST PressView users were either professional retouchers, very serious photographers like myself (I owned several) or people in print. But the point is, ColorMatch RGB is a color space based on the phosphor set and behavior of essentially an sRGB like device (emissive CRT display). Due to the custom phosphor set, the gamut was a bit wider than sRGB. Also, unlike sRGB, since at the time most users were doing this work on Mac (Photoshop didn't go PC until 2.0) and color management wasn't such that we had control over the OS, the gamma was designed as 1.8 rather than sRGB's 2.2. A side note, sRGB doesn't really have a gamma slope due to a slight tweak in the shadows. Anyway, the idea at the time was to produce the widest gamut possible for all needs and provide a way to calibrate the device to a defined target value and behavior. That behavior was ColorMatch RGB. When you finished calibrating the device, a message came up saying "your have calibrated your display to ColorMatch RGB standards" or something to that effect. IOW, you've produced a consistent working space behavior for use in Photoshop well before anyone knew what a working space was.

Point is, the display may have been marketed at prepress. Heck, they marketed it to anyone who could afford one. Only Barco had a more expensive unit. The color space is what it is, based on the characteristics of what a CRT could do back in the early 90's. It still isn't large enough to contain some saturated cyan's that are found in CMYK (SWOP).

Andrew Rodney