Why is my RGB to CMYK conversion causing a "dull" print?

Question

My black & white photobook looks quite dull out of print. I think I am misunderstanding RGB → CMYK conversion. The process for me was RAW to TIFF to Scribus (open source publishing tool) to PDF/X-3. With this PDF format, if I understand correctly, the RGB to CMYK conversion is done by the printer, with certain settings contained in the colour profile.

My display is not calibrated, nor do I have one that is suited for this task, really. I printed. I set sRGB colour space for the RGB photos, and ISO Coated v2 300% (ECI) for CMYK and printer colour space. The soft-proofing in Scribus looked quite dull, too, but I didn't know what to do.

So my question is, am I doing something wrong? I understand that the CMYK colour space is narrower than RGB but still, it can't be that bad.

Is a normal process for doing printing that you work in RGB, soft-proof in CMYK, realise that contrast is dull and then over-contrast the RGB while looking at the CMYK and send to print? I am using a print-on-demand service, by the way.

Answer 1

The basic answer answer is that a CMYK print can reproduce less colors that an RGB monitor.

But a more specific answer is that a CMYK conversion should be not used if you do not need it.

Manual CMYK conversion is specific for comercial printing, where the plates respond exactly as you define the values on the CMYK file; but almost all inkjet and laser printers make their own internal calculations-conversions. If you send a CMYK file they will convert it again, based on an already diminished file.

If you are using your home printer for example or an ink jet based system, leave your files as RGB all the way. In the given moment, the file will be converted to whatever conversion needs to be done.

Important. Here is a post for doing a basic calibration: https://graphicdesign.stackexchange.com/questions/71693/how-can-i-make-sure-that-my-on-screen-colors-are-consistent/71702#71702

But for exact results you should use a specialized hardware.

For working on the RAW file the best space is ProPhoto. But there is a chance you need to change it on the TIF or JPG exported files to AdobeRGB or sRGB. Make a test using the same file to compare.

Sadly you probably still need to make some tests, because you can not control or be sure the printer is calibrated, so send some test files and compare them to your screen.

But if they do not change the "calibration" you can find some consistent tweeks for that particular provider.

Answer 2

Starting with the display: If your computer display is not calibrated, you cannot be certain of what you are being shown by your display. Neither do you know what sort of colour space you are attempting to emulate. After calibrating the display, the printer should be calibrated to the identical colour space. The display is so vital that many people fit a hood to prevent unwanted specular reflections from falling on the screen.

Many calibration devices will calibrate the display and your printer together. I use DxO for RAW file adjustment and filing and Photoshop CC for image editing. I have used X-Rite Colour Munki Photo for many years without any issues whatsoever. I print at home to a Mitsubishi CP3800DW dye sublimation printer which is roll-fed and creates 12 x 8 inch prints. The only software step between the editing and the printing is Photoshop. I would avoid any steps which are (strictly speaking) not required.

When you do not align the printer calibration with the monitor calibration, It does not matter what the display shows to you, there can be no possibility of the printer producing what you see on your display screen, even if you believe it to be showing you the correct colours.

Monochrome prints commonly display metamers; where there is close but not accurate colour matching. Metamerism is where the observer perceives colours to be matching based upon the powers of spectral distribution. Unfortunately, the colours do not match and we see this in monochrome images quite frequently. As you look at the print, and move it into and out of the light, it appears to display a sheen of green or red colour, which is clearly unwanted and really not a true monochrome display.

The most widely understood CMYK colour space for commercial printing is usually known as process colour but more correctly it is known as the Pantone Matching System (PMS) and anything which you print using this system will be understood by your local commercial printer and your own printing device.

In image production work, it often helps to use the widest colour gamut you can because this gives you the most accurate translation of colours. ProPhoto RGB is the widest colour gamut while sRGB is the very smallest.

The following link will help you to follow this line of debate.

http://www.color-management-guide.com/choosing-between-srgb-adobe-rgb-and-prophoto.html

Your printer should translate the RGB image you can see on your display, into a CMYK image that matches what you can see as accurately as possible. You should soft-proof the RGB image and it will give you a good idea of how the printed image will appear. Some printing machines are unable to accurately work to a calibration and if your machine is very cheap, you may find that is causing inexplicable accuracy issues... But you will never know this until your monitor and your printer are calibrated to the same ICC colour profile.

The printing method you are using (calibration aside) appears to be unnecessarily convoluted. With every step (uncalibrated) you introduce errors into the final printed file. For example, you could save your RAW file as a PDF file and not have any intervening steps.

Questions for you:

• What computer are you using?
• What image editing software are you using?
• Why do you need to use Scribus for image editing tasks?

Summary:

1. Calibrate monitor to largest RGB colour space - ProPhoto RGB
2. Calibrate printer to largest RGB colour space - ProPhoto RGB
3. Soft-print image to display
4. Print image from display to printer if happy with soft-print
5. Save image as PDF file is you want to use it again.

Hope this helps.