conversion from rgb to cmyk black on every channels

[Fassara Sacko]

Hi ! Can someone helps me ? I've got a serious problem of conversion from rgb to cmyk. I often receive files (images and types) from clients made in rgb... they want to print their works in cmyk (separation process) from postscrip or any kind of process CTF, CTP... When using Adobe Photoshop, I check different channels... They are black in every channel. My concerns are with types. How to remove blacks on others channels but black channel, on the types side ?

 

[Eggy]

The next tutorial might be a bit elaborated but certainly useful when converting to CMYK

https://www.youtube.com/watch?v=UZc9TjRZjrA

 

[Tom Mann]

@Fassara Saco - I'm a bit confused by your question. If the files you are receiving are maximum black in all the CMYK channels, then the people that generated these files are almost certainly violating the maximum ink limit for a normal, full-color CMYK offset run, and if the printer actually prints them this way (on a normal full-color CMYK run), you and they will have a mess on your hands. Obviously, if they are doing single ink B&W printing, they will simply ignore the other channels and adjust the maximum black ink as appropriate for their press and the paper in use.

OTOH, if you go to the opposite extreme and make the C, M, and Y channels white, leaving the only blacks in the K channel, the result will likely be slightly faded looking blacks on a normal full color CMYK run (but, as pointed out above, NOT on a single ink B&W run).

Also, on a full-color CMYK run, on some printers (mostly inkjets using organic dye inks, not pigments, and not on offset presses doing a single ink run ), if only K is present, you may also run into problems with a slight color cast to the darkest tones, as well as metameric failure (ie, the black areas change color when viewed at different angles or in different lighting).

Anyway, if you could explain your situation in more detail, we might be able to help you. For example, are you talking about full color printing or just single ink B&W printing? Inkjet or offset? Spot colors, duotone printing or anything special like this, etc. etc.

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@Eggy - Nice find, Eggy. I do have a fairly serious disagreement with Varis, when, at about 15 minutes into the tutorial, he kinda brushes off the hundreds (if not thousands) of CMYK profiles that exist that are particular to specific combinations of printing hardware (ie, the presses themselves) and inksets, and instead, suggests that everyone use US SWOP. I have fairly strong feelings that as a major source of business for the printing houses, we should keep the pressure on them to generate and supply specific profiles for their operation. In the past 10 or so years, to save money, they have already off-loaded the pre-press work that they used to do onto us, their customers, whereas, in my opinion, they are clearly the people who know best how to do these conversions. Other than this general criticism, I did enjoy this tutorial as sort of a general introduction to pre-press preparation for newbies.


Tom M

PS - I am also confused by your statement, "... they want to print their works in cmyk (separation process) from postscrip or any kind of process CTF, CTP....". The reason is because the CTF or CTP step presumably occurs after you handle the image. Perhaps you meant to say, "... they want to print their works in cmyk (separation process) from postscrip TO any kind of process CTF, CTP...", not "or".

 

[Eggy]

What Lee Varis is presenting is his own vision.
But it is a start for converting to CMYK.

My knowledge in pro printing is rather minimal so when I do a work for family or friends I strongly recommend them to take the result of my work to my photographer/shop (since 1951 and thirth generation) to print the work and be sure to have the best result.
When I go for myself the son (thirth generation) always tries to explain me the right way (and sometimes it takes more then an hour and makes my head spinning) because he takes proud of delivering good work.
I was always satisfied with the results so...

 

[Tom Mann]

What Lee is addressing is CMYK for medium to large volume offset press operations. Most smaller volume printshop work (...say, up to a couple hundred copies) is now almost always printed on inkjets that are the same as what we might use at home, just larger ink reservoirs and faster. So, for submission to low volume print shops, all the same things that we do on our home printers pretty much apply.

Specifically, if you always send the job to the printer in sRGB, you will never be far off. If you want to get a bit more vibrant color for landscapes or graphics, just switch to Adobe RGB (for those inkjets that state they can handle it). CMYK is completely unnecessary for 99.9% of inkjet printing, as the conversion from either sRGB or Adobe RGB to the commands to spray various amounts of the 4 or more inks is always handled internally by the printer driver software that one installs to use the printer.

T