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reduce large CMYK file sizes

Just wondering if anyone can explain why CMYK image sizes are so much larger than their RGB counter-parts.

I ask because I'm working with our company logo in illustrator, and CMYK offers the most accurate match - when I convert to our "RGB values", the colours appear a bit darker, slightly different hue - so, I export the .ai as .jpg and use those files where needed (e.g. A4 document headers etc.). The problem is, a CMYK jpg (same dimensions etc.) is up to 8 times the size of it's RGB equivalent - so this can make documents very large (e.g. MS Word and .pdf exports). I've found a great work around - to create a template (for other employees to use), and to 'compress' the image once in MS Word through it's surprisingly effective compression tool. However, I've also provided the CMYK .jpg's as stand-alone files for employees to use when they're working on a unique document that may not already have the header and footer. So two questions:

1) How do I reduce the file size of a CMYK file so that it still prints accurately (and without any noticable compression)

2) Would this mean that my RGB colours need to be re-defined? Appreciate an RGB print is going to come out quite differently, but theoretically on screen, the RGB values should look very close to their CMYK standard (which was the base colour that we used when building our branding)

Any and all advice, much appreciated - many thanks!
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1 Solution
Alan HendersonRetired marine engineerCommented:
Just a thought.
If the CMYK profile is embedded in a file it can add up to 3MB to the size.
jon-bAuthor Commented:
yep - comfortably. That's why I'm wondering if there's a way to compress CMYK files for practical use (in printing and for screen display), without loosing image quality ;)

I am not sure about your approach. I have been involved in quite a few design & branding guides but we never started out with RGB.

You can make an easy distinction between your files, use RGB for any elements that are only ever seen on screen, use CMYK for all printed parts even inkjet or colour laser. You may want to have Pantone equivalents where you cannot use digital or litho print, such as Vinyl signs etc.

You would start out with pantone or CMYK colours for your brand, this gives you the most control over the colours you choose as the CMYK spectrum is limnited to what can be printed from these 4 process colours. In contrast RGB will always produce approximates from the CMYK files, eg if you use a C100M60Y0K10 colur and change the colour mode to RGB it will ghive you a different result than if I convert it here. You would have to add RGB to CMYK conversion profiles to make this uniform.

CMYK Jpgs don't display well in every medium, they are not designed for on screen viewing and therefore size does not matter. The reason your CMYK jpg is larger is, simply said, because it contains more data. A CMYK file needs to hold all information how the 4 colours CMYK will split out when you run plates from it and  contain separate adjustable channels.

So where do you go from here?

Best is to define all your images in CMYK, decide on the colours that come closet to your branding and convert all your images to follow that colour profile. Experiment with custome profiles in Photoshop, see:

Then you roll out your RGB files to your staff, that is all they need for there documents. As screen colours vary greatly between different setups, you have no control with your clients or customers see on screen anyway.

But your original files (PSD and JPEG) in CMYK will be the deciding ones when you want to properly print your stationery etc. as these are calibrated to your CMYK profile and you can use a Process Colour Guide to dictate to your printer what your colour should look like.

If you use RGB for printing you can only ever use digital anyway, and if you convert your RGB to CMYK and offset litho print these, your printer will not want to use them as you are likely to over-ink or use 4 colour blacks, as RGB to CMYK will use mixed CMYK inks rather than splitting to the 4 colours efficiently.

jon-bAuthor Commented:
quite true~ our base was CMYK, but I guess I was just hoping that there was a way of reducing CMYK file size, more than anything - there's no compression tools for this available? I'm sure there must be, MS Word is compressing the image inside the document/program comfortably, without loosing visible print quality...I just want to do this to files outside of MS Word~ Thanks!

Hi jon-b

MS Word is not a commercial print application, I suspect Word just embeds the file and applies its own attributes and discard anything it doesn't need. I don't think the JPEG is extractable with the same properties from Word, so that is not compression it is alteration.

I think the proplem you face is that you are trying to force CMYK files to be something they are not, a bit like if you decide to drive your car backwards from now on and then you try to make it go faster in reverse.

I have not heard of external compression to JPG files, the jpeg format is probably the most efficient compression format there is for images. But as said before you are not using the best methodology if you use CMYK jpegs anyway so your answer lies in modifying your approach.



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