This is an update to a problem where a digital RGB file was converted to the (printer's suggested) US Web Coated (SWOP) V2 CMYK profile for printing as a postcard. Over the print run the neutral background colour did not stay neutral but varied unacceptably to three different shades from geenish through neutral to magenta-ish. In an effort to understand why, I put the question to the list before contacting the print company. Stephen Marsh has worked with me in the most helpful and instructive manner, to find out what the problem was and see what could (have) been done to avoid this end result. Our correspondence (condensed) follows.
Several people contacted me offlist and asked for this, and the acting Listmum suggested that I post for the benefit of all interested. His actual action is not included, though his explanation will be enough, I'm sure. The image - RGB, reduced in size and converted to jpg for the web - is to be found here: http://www.iistudio.com/cmyk_problem.html (copyright Steven Kennard 2004) It appears that the fault is divided between the print company and myself. In my case it was ignorance as to the fact that this was an unusual image that needed special treatment. This particular image needed two different types of separations to allow the pressman to hold a neutral. Stephen Marsh said: SM: If a neutral tone is formed mostly of CMY with little K ink - it is harder for the pressman to hold a neutral. On the flipside, using more K in a coloured job can also have issues, but in neutral grey areas it is often a good thing. This job could have used higher GCR than is offered in the SWOP v2 profile (TR001 colourimetric data). This uses medium type GCR in this Adobe profile. I see two issues with this image: a) The background which should be neutral. The CMYK file you supplied has very good and traditional neutral CMYK values, as in the CMY are good for greybalance for TR001 conditions...if the printer can hit these conditions with the stock/ink etc. It sounds like they did in 1/3 of the cases getting corrct neutrals and correct brightness. b) The main image itself (not the background) You would probably need to separate the "hero" differently than the background. In essence this image can greatly be enhanced by using two different separation methods and combining them, taking the best results of each conversion and masking them together. A heavy GCR separation for the gradated netural background and a lighter GCR for the main image content with special attention to the K plate. With neutral grey balance on average being formed from + 50c40my with 10K - the bias is on getting the CMY correctly in balance for the background. More K in the background would have been better, but NOT more K in the image! Thus combining two separations into one. EK: Were my expectations too great? SM: Yes and no. For the GCR ratio that you supplied and the image content - yes. In your general expectation - no. It needs to be understood that what the aimpoint is...have you communicated your desires correctly and are they understood? Is the price of the job going to warrant first class work? Some printers gang up multiple jobs to save costs (bulk business card or post card printers etc) - your job is one of many on the entire press sheet...due to the mechanics of offset print large amounts of colour can affect the draw/flow of ink to other areas of the press sheet. The colour design of elements on one page or the inclusion of other pages may alter the content in other areas. Is it a neutral background in all cases at the expense of the main image? Is it the main image at the expense of the background? Should both the main image and the background be reproduced ideally? Prepress is my trade, I have been doing it for the last seventeen years. I find it sad that now that anyone can do their own prepress, my craft has been devalued a lot. For many images and jobs, an acceptable separation can be performed with little forethought of the original and the destination and a simple push button approach can be performed. But some other images are best handled with experience and human analysis - which is missing from a profile transform which does not know anything about the image. EK: I shall go to Dan Margulis' book (the only one I have that deals mostly in CMYK) and see if I can make more sense of it. SM: He has a section on this about GCR and neutral tones under 50% brightness...which need heavier GCR, just like I am talking about and demonstrating with this action and emails. EK: Does this mean that, in your opinion, I should say nothing to the print company and realise that this was entirely my ignorance on how to correctly treat this image for press? Perhaps this is just a learning experience for me? SM: Hmmm... They did recommend a profile which did not deliver enough GCR for the image content to give the press operator a reasonable chance of holding consistent neutrals and tonal brightness over the print run. If they saw the image first and they knew their game, they would have noted that the image is not typical and perhaps should not be handled typlically - or that different expectations may be set between all parties etc. So, if you can get a 2/3 or 1/3 free reprint of the existing data/separations to match the prints that you are happy with...then that is good. Also chalk it up to experience as well. Both parties have equal blame/responsibility in my opinion. If they are willing to learn from this as well as you, they might wish to see what a new separation provided by you prints like using methods suggested my myself...this may educate them and yourself and provide a good test piece or education piece etc. You might be able to negotiate something here. Presswork really is a crapshoot, a gamble. The idea of the knowledgeable prepress operator is to second guess the press issues and provide a separation which will aid the press operator. The problem is that it can be hard to get a good answer from press guys as to what is good or bad until ink hits the paper - many are good at evaluating files or separations but many are not and prepress is often more experienced at these decisions. The steps he used to get the image optimised for the press follow: * A base conversion is performed to generate the correct separation for the hero or foreground object. In this case I used the Chromix UCR TR001 profile instead of the Adobe TR001 SWOP v2 profile as I prefer the conversion for this image as it has a contrast kick in the perceptual intent whereas the Adobe one does not (this is not too critical). Using either TR001 profile would be OK here. * A dupe of the RGB is also made and converted using Photoshop's legacy Custom CMYK engine using special custom values to generate a heavy GCR separation which is close to TR001 aimpoints. By default custom CMYK does not match the same grey balance as TR001 even though it has SWOP in it's name. As one can't edit the Adobe v2 or Chromix profiles in Photoshop to create heavier GCR one must use the old custom CMYK to generate the required amount of black in the four colour grey neutral background. * Both CMYK files have the same SWOP v2 profile assigned to them, as they are both separated for the same print condition (they just happen to use opposite black generation methods!!!). * One CMYK file is layered over the other. One then masks the image so that the file with heavy GCR has the foreground hero image hidden/masked. This is the upper layer above the lighter UCR separation that is good for the hero image. * A mask is automatically generated based off grey density using the layer option blend if sliders and also using a layer mask generated from the K plate. (Stephen then corrected himself with a second action and this explanation:) I was not happy with my layer option blend if sliders used to mask the image - the manual layer mask is better but must be refined by hand. This new action resets the upper layers blend if settings to default so that they do not contribute to the masking effort...now only the layer mask is used and the layer mask obviously needs to be tracing the foreground object well. Many thanks once again, Stephen! Ellie -- Ellie Kennard, Innovative Imaging Studio : http://www.iiStudio.com Contributor to "Photoshop Restoration and Retouching, 2nd edition" by Katrin Eismann =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
