U of S | Mailing List Archive | alt-photo-process-l | Re: cmyk negatives?

Re: cmyk negatives?

It's very simple to convert from RGB to a blackless CMY(K). In PS, go to Edti>Convert to Profile, then Destination Space pick up Custom CMYK. On the dialog, select SWOP, Euroscale, whatever; write 0 (zero) on Dot gain, Separation Options choose GCR, and on Black Generation choose None. Give a describable name and click OK, and you're done. The only problem is that you're gonna miss the great advantage of the K channel (because the CMY channels will very likely be similar to their RGB correspondents, only muddier), which is to enhance the details.
Rodolpho Pajuaba

Katharine Thayer wrote:
Hi Charles,
The short answer: leave your image as a RGB image; invert, separate the channels, and print.

For more detail about why that's the best way to do it (IMO, of course) and how RGB inverts to negatives that print CMY, if you're interested, see this page:


For more detail on why using the default CMYK space may not be the best option, especially if you discard the K layer, see:


The short answer is that the default Photoshop CMYK is designed for printing SWOP process inks on coated paper on a web-offset printing press, and when you convert the RGB image to CMYK, the color information in the RGB channels is altered, in some cases drastically, to accommodate the addition of black without changing the brightness value of the pixel. That's why the CMYK image looks flat without the black, because the color information isn't the same as the color information in the RGB space.

On Aug 25, 2007, at 5:03 PM, ryberg wrote:

Folks, I've finally worked up nerve to try a three color gum. I have made curves for yellow, cyan and magenta which look like they might work. My problem is how to make the negatives. I thought it would be simple--convert image from RGB to CMYK, separate out the channels, invert, print each one and that's it. Well, first off, without the K channel the image on the screen is really flat, but I sort of remember that few of you actually print a black layer--am I wrong? Any advice is appreciated.
Charles Portland OR