'Safest' way to convert PMS - CMYK

Question

Hi, I'm wondering what the 'safest' or most reliable method is for converting PMS spot colors to CMYK when we might not know the end printer?
I was reading up on the difference in the conversion in InDesign (and Illustrator) when checking the "Use Standard Lab Values for Spots" box in the ink manager. So I understand how they come to different conclusions (sometimes pretty radically different) but don't really understand which is 'better'.
I know this is a bit of a subjective question, but would love to hear from folks who have experience here.
Most of the time, we are not aware of the printer who will end up printing a certain piece. And in the cases when we do actually know, I've found that the majority of the printers I've spoken to don't have a custom profile for their press (or don't know what that is) and usually tell me they calibrate their presses to SWOP standards or something.
For the current project, it's going to one of those large gang-printing shops (4by6). They state that they don't recommend using ICC profiles and that using a standard SWOP2 profile would give an approximation of their presses.
- So if all we know is 'use SWOP', and I have my document set to use US WebCoated SWOPv2, in general would using the LAB conversion give me a safer match or trusting the Pantone conversion?
(The numbers are very different, for instance our dark blue PMS 539C is either 100C, 49M, 0Y, 70k using the Pantone conversion, or 100C, 77M, 47Y, 51K using LAB conversion. Those are pretty radically different numbers and kind of make me nervous).
THANKS!!!

Answer 1

Jethro,
-  And for the sake of understanding, if we're usually printing on  uncoated stock but WITH a U/V coating, would coated PMS values be closer  than uncoated in this scenario?
I do almost all coated stock, so I can't really answer that one. I suspect things will fall somewhere between uncoated and coated if using UV, but that is merely a guess.
But IN THEORY, would this be the order of  preference for conversion?
1) Use the LAB conversion with accurate press  profiles provided
2) Use Pantone Bridge PC conversion
3) Use the  older Pantone CMYK conversion (default in InDesign and Illustrator)
My experience has led me to choose different methods depending on a number of variables. (Sorry, I know you are looking for a simple, easy answer that fits all scenarios).
If I was printing on uncoated stock with a low total ink limit of 240–260, I'd be inclined to stick with a solution that reduces total ink load. For dark colors, this means more black ink (heavy GCR). This will result in less color shifting on press, but may also have a tendency to have a little less vibrance than a light GCR. But, it definitely reduces ink load, which may be a big consideration, depending on your final paper, press, ink.
CMYK is widely known to be deficient in reproducing blues, and there can be a tendency to shift toward purple, so I am always cautious if there are important blues in the job or if they represent a large area. I will select the method that tends to err on the side of less magenta, rather than more.
Light Pastels are another tough challenge for CMYK, especially if printing on a dingy, uncoated stock that has any yellow bias. The only way to get those light pastels is to spread out the dots and make them small, so the paper exerts a big influence on color, saturation, etc.
Another factor that weighs into my decision is the commercial printer. If I don't know who will print the job, I tend to be very defensive, pick a middle of the road standard profile (I've been usning the IDEAlliance SWOP2006_Coated#3v2 profile for my coated work). The final press may be able to utilize 320, 340 or higher TIL, but if I prepare the file that way, and it heads to a 280 or 300 TIL press, I'll have a problem. I also strip out the profiles to prevent unwanted conversions and hope the printer comes close to SWOP G7. If I am working with a color managed printer that I know and trust, I get their profile and design the job around their press and specifications. In that case, I feel more confident pushing things a lot farther, knowing that my hard proofs will match theirs (from experience with that printer). I still err on the side of caution with blues. The Solid to Process uses way too much magenta in the blues to suit me, so I'd use the Color Bridge or Solid Coated books for blues. I also prefer to convert all my images to the final CMYK space in Photoshop instead of InDesign.
The easy answer…I'd probably use Color Bridge to select my colors, since it is the latest guide and seems better with blues (at least to my preference). If I had important colors I had to hit, I would do an accurate hard proof and make sure the colors are acceptably close.
Using Lab is fine, but which Lab value are you going to select? The Lab values from different swatch books are different.
One final thought—CMYK is unlike RGB, HSB, LAB and other tri-stimulus spaces. With three coordinates, there is only one way to define a given color (assuming you have nailed down your working space...sRGB, Adobe RGB, etc). With CMYK, you have four colors to work with, and can create the same color with many different combinations, and they may all print exactly the same.
Lou