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When exporting InDesign files as PDF, it is converted usually to the intended colour space. But, what happens when you don't do that for whatever reason and send the non-converted PDF to print?
Since the printing ultimately is in CMYK, wouldn't it eventually have to convert the RGB colours to CMYK? Or is it that the printer wouldn't even accept such a colour? Then how does converting beforehand help?
What happens if you are sending it to a comercial printer (offset) is that they will not accept the file. An error like that cost money and they will not print it.
Then how does converting beforehand help?
It does not help. It is mandatory.
wouldn't it eventually have to convert the RGB colours to CMYK?
If you are refering to a digital print, the driver of the printer makes the conversion.
Some printers need to make a conversion to CcMmYK so it is better to leave the file as RGB.
It is converted usually to the intended colour space.
It is converted usually to the intended Color Model with the intended Color Profile working inside the defined Color Space.
The color space is for example Adobe 1998
The color Model is CMYK
The color profile is for example SWOP2 coated, or Fogra27.
It would depend on the printing method.
It wouldn't work for CMYK/Process offset lithography - since CMYK separations are required to make printing plates. The print job would be rejected, and your printer would ask you to redo the artwork in CMYK, or they'd charge you extra for the trouble of converting it for you.
For digital inkjet/colour laser printing, the printer would handle the conversion to make it printable.
I will answer your last question
Then how does converting beforehand help?
First - you see what the colors will look like. Not exactly the same as final product but close.
On the left are colors in RGB on the right are CMYK. Add to that the fact that paper can make it more dull and your fresh beautiful green grass may turn into brown pile of brown pile.
Then when you are converting colors you can (and probably will) experiment with different profiles. Depending on the paper, printing machine etc your outcome may differ. YOU will choose what best suit you and what you want to achieve.
Why you should do this? Because then you can print Cromalin or "Proof". Something that will show the colors after printing. This will save you time and money when talking to printer or when making a complaint.
Sometimes the printer will use profiles build in their RIP. They are often set to change CMYK to machine CMYK (for example the machine will add 10% of Cyan so the profile take that 10% from plates). Unfortunately those RIP profiles can ignore strong RGB colors (as they are set to convert only CMYK to CMYK). I've seen prints where almost Green or Blue objects where just very light grey.
Converting beforehand give you also one, often overlooked, profit. You will comply with the Total Ink Limit. Some printers will just ignore the fact that your work is very dark and have almost 400% TIL and will print it. It will
A) warp the paper
B) will need powder to absorb the excess of ink.
It will of course, make printing time take longer and the Printer will bill you for the extra time on the machines, extra work needed to handle the prints and will tell you it's your fault.
