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RGB to CMYK Best Results


S

SamSlam023

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Hi all,

I am getting an enormous poster printed for my friends birthday (1.4 metres wide!) so I need to get this picture looking its finest.The image is currently in RGB format. The place I am getting this printed suggests that I convert it to CMYK and then change it to 100% black and 0% cyan, 0% magenta and 0% yellow for better results. This apparently "stops too much ink being placed in the one spot for black." I have very little experience with colours but when I convert to CMYK (in Photoshop CS6) I can't seem to be able to find where to change the black to 100% etc.

Where can I do this? Should I just leave it as it is now (just CMYK no adjustments) or will this make a difference? Is there anything else I should play around with to get the best out of this picture?
 
also I am getting it printed on B0 paper (1000mm x 1414mm)

Should I change picture to a specific resolution? At the moment it is 1920x1080. Printer resolution is 150dpi and 5mm borders. Would this mean I make my picture the size of the full page (including the 5mm) and that will just get cut off? Or do I have to compensate for this?
 
"The place I am getting this printed suggests that I convert it to CMYK and then change it to 100% black and 0% cyan, 0% magenta and 0% yellow for better results. "


You probably misunderstood them. Specifying all 4 values is the recipe for one particular color. If you make the entire image have that one color, you are going to have one extremely boring poster. :eek:

Also, these days, no printer that deals with the general public would ever ask a customer to perform the RGB -> CMYK conversion. There are too many variables involved. 99% of the time, a printer can perform this conversion better than the customer and prefer to do it themselves, adjusting arcane parameters to match their particular press, inks and paper.


T
 
Last edited:
A few more thoughts ...

re: "... This apparently "stops too much ink being placed in the one spot..."

This sounds like they are describing an adjustment called "dot gain" that controls this. Again, the proper setting depends dramatically on exactly what combination of printer, inkset and paper is being used.


re: "... Should I just leave it as it is now (just CMYK no adjustments) ..."

To be honest, if they refuse to take an RGB from someone who obviously isn't in the trade, go somewhere else. There are a huge number of firms that will make a poster-sized print from an RGB file for you. These include all the big on-line and walk-in services such as Mpix, Shutterfly, Vistaprint, Staples, Office Depot, etc., plus mail-order operations that specialize in posters, e.g., shortrunposters.com.

re: aspect ratio issues

As you correctly observed, if your original 1920 x 1080 px image is simply scaled up to 1414 mm in the long dimension, then (ignoring the small border), the short dimension will be only 795 mm, which is a good bit narrower than the 1m of the poster.

You have four choices:

a) Add white space (or any color you want) to the short dimension so that it comes out to be exactly 1m when scaled up;

b) Crop a bit off the long dimension so that when the short dimension is scaled up to be 1 m, the long dimension is 1414 mm;

c) The difference in aspect ratio between what you want and what you have is not extreme, so you could probably scale the two dimensions slightly differently and most people wouldn't notice (especially if there are only graphics and text on the poster); or,

d) Most printing shops will do either (a) or (b) for you, so let them decide and make an appropriate decision.


Perhaps what you should be more concerned with is the fact that the starting pixel dimensions are quite low for a poster of this size. This means that even using the best available up-rez'ing software or RIP, your poster will be somewhat soft. This will be very noticeable if someone looks at it from a couple of feet away or less. However, if the poster is only going to be viewed from several feet away or further, most people will not notice this softness. It's your call whether you want to use this image and accept a bit of softness or use a different image.


HTH,

Tom M
 
The place I am getting the poster printed only recommends CMYK colour mode. They do accept RGB. From what you said I think I will send it in as RGB and let the printer do the work for me.

Should I let them do the upscaling or would I get better results scaling it up myself? There seems to be many ways to do this, what method would you recommend using (preferably in CS6)? Lastly what resolution would I upscale it to? 55.67(width in inches) * 150 (dpi)? Or should I just make the resolution bigger than what it needs to be and get it downscaled?

Lastly, I don't NEED crop markers do I? Could I just say to get it cut by x amount?
 
SS, I see you started a new thread to separate out your up-rez'ing question. I'll respond over there.

Tom M
 
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