CMYK Conversion

[Greg Miller]

I don't understand the comment about using "relative colorimetric" instead of "perceptual" as the rendering intent.

Relative colorimetic shifts all out of gamut colors to be in gamut (to the nearest color on the edge of the gamut shape. Colors already in gamut are unchanged. (You may see some banding in the out of gamut colors afte they are shifted).

Perecptual shifts all colors until all colors are in gamut (shirninking the gamut of the image until it fits into the printer/paper/ink gamut shape.

In either case, all out of gamut blues will be shifted until in gamut. The image will appear different in both cases but you aren't likely to retain the vibrancy of blues in either case.

A rendering intent of "Saturation", on the opther hand, may work. This rendering intent will maintain saturation by shifting out of gamut colors to the nearest color that can maintain saturation. The downside to this is your blues may shift to a completely color (i.e. green).

[Marko]

I don't understand the comment about using "relative colorimetric" instead of "perceptual" as the rendering intent.

Greg, it's just like you said: Relative Colorimetric shifts only out-of-gamut colors until they fit into the target space, while Perceptual shifts all colors, regardless of whether they are already in gamut or not. The resulting tonal ranges are very different, with the Perceptual one looking flat in comparison. It has to look flat because it compresses all the colors proportionally, while RC compresses only the colors at the edges but far more sharply.

As for Saturation, having the blues turn to greens in a photo wouldn't quite qualify as "working" in a practical sense.

Regards,

[Greg Miller]

Marko - Kirk said he was concerned about his vivid blues getting clipped. Perceptual will decrease saturation ("looking flat") which will make his problem even worse compared the relative colorimetric.

[Marko]

Greg - well, yes, that's why I suggested using Relative Colorimetric instead. Perceptual is usually a default setting, which many RGB users, such as photographers, rarely if ever change or experiment with.

Being one of those myself, I was never aware of the difference until an AD in one of the magazines I worked for pointed that out.

[Paul Schilliger]

I'm not a color guru, but I can identify with your disappointment more than I wished. One question that is often overlooked is to know what settings will be used by the printer to convert the RGB file to CMYK, and many printers don't even know what takes place in their system! Some say they are profile savvy when they are not! I have had the most disappointing experiences when I sent Adobe RGB files. Sometimes the Adobe profile was simply ignored in one of the steps, can be the page layout or CTP process, and the result was a poorly saturated image. Now I almost always send sRGB files, so if the profile is ignored, the image will keep most of it's saturation. It certainly wastes some of the fine tones and hues, but it's foolproof. Sending a CMYK file makes no sense unless you know what profile the press is calibrated for. The printer should tell you, but my experience is again that the answers can be wrong or vague. Once (if!) you know what profile you should use for the conversion, you can use the Photoshop preview and "out of gamut" command to detect the colors that wont be printed correctly. It doesn't do miracles, but you can tweak them until you find an acceptable compromise. Here is a link that should help get the idea:

http://www.udel.edu/cookbook/scan-print/gamut-hue-sat/gamut-huesat.html

And of course you should always have a match proof signed by both parties to draw the line.

[Andy Eads]

Kirk, The problem may be all of the things mentioned before plus a combination of ink, paper, press and skill at the print shop. Not all inks are created equal so merely converting to CMYK is meaningless without the print shop sticking to a published standard such as SWOP or giving you their profile for the ink/paper/plate/press combination they are using. If they can't give you clear guidance, they don't know themselves. Texas Architect by contrast has a vested interest in the best reproduction quality and has invested in training and equipment to handle good image files.
One other thing I don't recall seeing mentioned in this thread is setting PhotoShop to emulate the final print process. This is vital so you can get a soft proof of your image to determine if the colors can be reproduced well. Dan Margolis has written some fine articles and books on how to squeeze good color from CMYK presses.
As an aside, I teach high school photography and I have my kids visit your site when I'm teaching architectural photography.

[Kirk Gittings]

Thats great Andrew. It is comforting to think that my work might inspire some young artists.

[Greg Miller]

Marko - sorry - I got twisted up in whic intent you were recommending. I'm still thinking that the difference will be minmal since the out of gamut colors will get clipped either way.

[milepost206]

Hi,

"Sending a CMYK file makes no sense unless you know what profile the press is calibrated for."

I can offer my experience with CMYK. The above statement is not quite true. It would be nice to have the current press profile used by the printer, but what happens when they re-profile the press the day after you send your image?

However, with the press profile you could generate your own proofs of what your image may look like as long as you profile your printer.

So for the same reasons you calibrate your scanner and monitor, the press profiles are used by the printer to provide that baseline to use when he converts your images developed with your profile are converted to the profile he has for his press.

Most CMYK problems seem to occur because the printer does not profile his press and have that file loaded into his image program. If he does, it is close to a no-brainer.

A lot of printers are just learning about color management and how to implement it in their workflow.

But keep in mind RGB image files can be a printer's nightmare and only progressive printers profile their presses and most use direct-to-plate technology that demands accurate file conversion.

Another part of the problem is providing that image in RGB format. You are relying on the skills of the unknown and their use of color management to manipulate your file.

I recommend you convert the image to a CMYK press profile before you submit the image. If you must use a RGB image I think you will achieve better results if you start with a sRGBIEC61966 2.1 profile. And without starting a purposing war, it really might be better if you scan the image in CMYK to begin with as long as everything is profiled.

The next potential solution is to use a color management program that generates a true Photoshop profile. Most say they do, most don't. They generate an ICC profile.

PS cannot use all of the data from a full ICC profile. Don't ask me to explain why, it just can't. This too, can affect the image during conversion from RGB to CMYK. You can see the differences in conversion when you have a PS profile loaded and do a conversion. It is as close to seamless as it gets.

We have found a profile that works very well for high quality (175 lpi) sheet-fed, offset color. It is OffsetEuro U340 K96 V25PO4. We use Offset SWOP neg Gray PO4 GRAY for all of our B&W. Most magazines are printed on a web press.

If you have all of your images in one profile, it makes it easier on the printer, too.

These suggestions do work even if your printer doesn't profile his press. They are suggestions. I don't recommend using the PS canned SWOP color profiles. They are too generic to be much good.

All of the work you do to achieve exceptional images can be ruined faster than you can imagine by converting from RGB to CMYK.

Control your own destiny.

Randy

[Gordon Moat]

Teach me something here please.

. . . . . . .
See www.gittingsphoto.com/fmsetgallery.html?gallery=New%20Architecture%20Projects 6th image down as an example of an image I have had problems with.

. . . . . . . . .The blues in these reproductions were clipped significantly vs. files I had supplied other magazines as RGB files. Same images. What am I missing here? What am I doing wrong when I convert to CMYK vs. magazines which are doing the conversion from my RGB files?.

Hello Kirk Gittings,

Very nice image there. I also do some night images, and happen to have started as a pre-press and printing specialist after I graduated with a Fine Art degree in 1998. Basically, there is no one answer to some very saturated colours that would work every time when applied the same way each time. While a somewhat standard workflow can get you consistent, going that extra step to something really eyeball busting takes extra work on a case by case basis.

So one of the rarely used choices in PhotoShop would have worked for this image, though it would have left you needing to adjust in CMYK. This is something most gurus and workshops avoid, mostly because they are RGB biased. Even the printing industry, when I go to trade shows, Print Week, or any of those seminars, largely feel that the professional photo community is stuck in RGB and does not understand CMYK . . . sad situation that I hope will change in the future. Those who might feel like blasting me for such statements, please PM me instead . . . thanks in advance (not being punchy, but I always seem to get flack for suggesting photographers might want to understand CMYK and commercial printing).

Marko sort of had the right idea with the Relative Colorimetric suggestion, and that is an excellent choice for the majority of images. A more radical approach (unless you have done this more than a few times) is to use Absolute Colorimetric, do no adjustments in RGB, and do all adjustments in CMYK. This clips the tones upon conversion, though you can then edit in CMYK the individual channels to get the balance closer to your original. Warning: this is something that should rarely ever be used, and you have to understand CMY relationships to get the balance correct.

Magenta ink is unfortunately quite strong, while Yellow is very weak. That deep blue sky can be accomplished with just Cyan and Magenta, but if you get too much Magenta, then it will look more purple than blue. Obviously, each press will be slightly different on how the percentage mix of these will render a deep blue sky. You should definitely do hard copy proofs before getting too far, since your mixture of Cyan and Magenta will often not appear propery on any computer monitor.

Perhaps and easier way to do this is to create squares of colour in PhotoShop, using 100% Cyan, and various percentages of Magenta. Then you can test print the combinations to a CMYK proofing printer, and find the combinations that work best. Printing places try to achieve to SWOP standards, though many can do much better than SWOP specifications. There are also coated papers that don't handle much total ink, and uncoated papers that can take large amounts without warping. SWOP sheetfed coated is one choice that can work surprisingly well with many magazines, even those run on a web press. Some magazines, like CLEAR out of Detroit, are using better paper and printing than many brochures or posters, just to state one example.

I wish I could go more in delpt, but I don't want to type too much. Please, please please, feel free to ask more questions. I wish there was just one greatt solution workflow that always worked (100%), but some images just will not fit into easy solutions. If you want to look up a controversial figure who writes well about CMYK, LaB working space, or other odd uses of PhotoShop, check out Dan Margulis:

http://www.ledet.com/margulis/

I will warn you ahead of time that he is highly criticized. Judge for yourself whether he makes sense. Personally, I find some of his information very useful, though I disagree with some things, especially his LaB working techniques. Anyway, I don't ever want to be just one source, so I encourage you to find others. Best of luck.

Ciao!

Gordon Moat