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In photoshop converting a file from RGB to CMYK is a simple


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Because – simply put – drop-down-change-to-CMYK ist not what persistent color management is all about. Since there is something like profiling colors since about the year 1998 you should never simply change images the way you described it. 

 

The question goes deeper: why would you change RGB files to CMYK anyway? Nowadays layout apps usually can apply the specific profiles on output, so you don’t have to hassle with 2 versions of the same image anymore, one for print and one for web. If you talk to your local printer / litho guy they will most probably go like "fine, bring us RGB files, we can easily use those for our purpose."

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If you are outputting to PDF, you can do the CMYK conversion as part of the export. The PDF/X profiles do this automatically. PDF/X-4 keeps the images as RGB, leaving the final conversion for the printer to do, which usually gives better results if they support it (because they know their inks).

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In photoshop converting a file from RGB to CMYK is a simple drop down selection.

Why is affinity photo unable to do this?

 

How can I convert from rgb to cmyk for commercial printshops for litho repro?

You really want to say, you use this "simple" menu command for converting your image for "litho repro" in Photoshop? Seriously?

This is so simple, because it is the wrong way. For quality printing you definitely have to use the profile conversion commands.

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Peterkoasa or mac_heibu...Could either of you be more clear on why using the drop down menu to select CMYK is not a good idea. I have clients that require all files be CMYK and I routinely convert from RGB when needed, using the drop down menu in photoshop. I have never had a problem and I can see immediately if there is a significant color shift that needs addressing. I am far from an expert on this and use the drop down because I didn't know there was a better way or a way that should be used.

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This is not a good way. Using this "simple" conversion – into which profile it will be converted? You even can't really use the soft proof functioality, because you don't know the profile. You don't even know the ink coverage, which is crucial for quality printing

Normally you should use the command "Convert to Profile" because this is the only way to respect the required color profile (in Europe often ISO Coated, in USA Web Coated and so on; many print houses require certain profiles. For example "OnlinePrinter" wants "ISO coated v2 300.)

In short: CMYK isn't CMYK just as RGB isn't RGB. Simple converting RGB by using the above mentioned command is like going to a book store and say: " Hello, I want a book. Do you have one?"

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mac_heibu, I appreciate the response, and I'm sure it's me, but I still don't understand what you are saying. When I convert an image from RGB to CMYK using the drop down menu and then save the file, the color profile U.S. Web Coated(SWOPv2) is saved with the file, which is what I think Magnus is referring to, so I don't understand how this is a problem.

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Are you American? Does you print service requires this profile? If yes, don't forget: The world consists of more countries then the USA :)

It isn't correct to simply recommend the procedure you use. If your print shop needs Swop v2, you may do this. If not (like in most parts of the world) you definitely shouldn't. And most users aren't aware of what they are doing by simply pushing the CMYK button. They don't know, that they use the US standard swop v2 "under the hood".

If you use print service, which performs fast (digital) printing, the swop profile has a much, much too high ink coverage of about 360 %. These printing machines definitely need an ink coverage of maximum 300 %.

And one more thing: If your printing servive needs for example ISO coated but your document is WEB coated, they have to perform a CMYK to CMYK conversion, which in most cases ends up in minor quality and 4-color-text (should only be K=100).

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mac_heibu,

 

What are your thoughts on this "Preparing images for press" article?

Affinity Photo 1.10.5, Affinity Designer 1.10.5, Affinity Publisher 1.10.5;  2020 iMac 27"; 3.8GHz i7, Radeon Pro 5700, 32GB RAM; macOS 10.15.7
Affinity Photo 
1.10.5.280 & Affinity Designer 1.10.5 for iPad; 6th Generation iPad 32 GB; Apple Pencil; iPadOS 15.0.2

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It is ok, with one exception CMYK to CMYK conversion is not really a good idea, if your image contains black text.

For printing purposes black should be K=100. Multicolor Black could cause issues, especially for small text, because these print colors can't be precisely printed one over the other. CMYK to CMYK conversion produces necessarily 4-color black. The only way to avoid this is by using DeviceN profiles, which aren't widly used at the present.

The best way to convert images to CMYK definitely is using the Convert to Profile" command, setting text after conversion (if it has to be done in Photoshop), but having done all color corrections before converting. Doing corrections after converting could end up in a "lie": You say, the image is, for example "ISO coated 300", but your corrections (e.g. adding more contrast) pushed your ink coverage far over the 300 % limit.

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mac_heibu, you clearly have a much deeper understanding of print requirements and color profiles than I do. The method I use works for my needs. I did not choose the CMYK profile by accident. I don't use four color black, spot colors or pantone colors. I keep everything as simple as possible and have not had a problem so far. Fortunately I am aware that there are more countries in the world than the US. Now that I know about the Convert to Profile command I will keep it in mind should I ever need to send something internationally.

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