Photo purchased from … www.depositphotos.com
I learned something today. A colleague of mine emailed me saying her book designer planned to save all photos in RGB rather than CMYK format. My colleague asked my opinion. So I did some homework.
I have been using Photoshop since the late ‘80s or early ‘90s. Back then any automatic translation by the commercial print provider from RGB to CMYK was rather problematic, yielding unpredictable and sometimes muddy results. So I, and all the designers I knew or managed, always changed photos from RGB to CMYK when preparing files for commercial printing.
RGB vs. CMYK
First of all, let’s define some terms. RGB (red/green/blue) is the color gamut used for online presentation of art, photos, and text (i.e., anything related to the combining of light rather than ink). Red, green, and blue phosphors create all visible hues on the screen. These are called the additive primaries. In contrast, when you prepare files for offset commercial printing (or printing with any other kind of ink), you use cyan, magenta, yellow, and black ink. These are the subtractive primaries (actually only cyan, magenta, and yellow are the primaries; black is added by printers to darken and give definition to the resulting CMY ink mixture).
When you combine the additive primaries on a computer screen, you get white light. When you add the subtractive primaries using ink, you get black ink (closer to muddy black, but that’s why printers add a separate black ink, as noted above).
Look inside a color laser printer. You’ll see cyan, magenta, yellow, and black toner cartridges. Look at an inkjet printer, and you’ll see cyan, magenta, yellow, and black liquid ink cartridges. Truth be told, if you look at a high-end inkjet printer (like a grand-format flatbed printer), you’ll probably see other colors as well, such as red, green, and blue; or orange and green or purple; or light magenta and light cyan; and maybe even a few different black ink mixtures.
In all of these cases, we’re talking about augmenting the CMYK “color space” when putting toner or ink or any other physical rendering of color on paper or any other substrate. This is in contrast to any colors made with light (even theater lights with gelatin color screens over the bulbs would fit into the RGB model rather than the CMYK model).
So why is this important? How does it relate to your daily design work in Photoshop and InDesign?
Most importantly, the RGB color space has a much wider color gamut than the CMYK color space. You can produce significantly more individual hues. Even though you might want to print in RGB format (but you can’t), you do want to keep photos in RGB format for as long as possible before the final conversion to CMYK prior to commercial printing.
A Wider Color Gamut
At this final transition, by the way, colors in the RGB color gamut that can’t be reproduced via CMYK custom printing will be converted to the nearest printable color. In the late ‘80s and early ‘90s, this transition was less than perfect. Now it is actually more skillfully done by your offset/digital printer’s software than by you.
If you make the change yourself, you can do it in Photoshop, or you can make the change in InDesign when you distill final press-ready PDFs for your commercial printing provider. But if you let him make the change, he can optimize the photos for his own press and for the specific paper (gloss coated, uncoated, etc.) that you’ll be using. This is a lot more precise than just going to the mode command in Photoshop and changing the image from RGB to CMYK.
Custom printing presses also make a difference. If you’re still in an RGB workflow at the last possible moment, it will be easier to optimize output for either web-offset printing or sheetfed-offset printing. If you leave this transition to your commercial printing supplier, you never have to change the photos themselves.
So while it’s understandable that I (and my colleague, who is the same age) and probably a lot of other designers who started working with PageMaker, Illustrator, and Photoshop (30 years ago) would (entirely out of habit) automatically want to convert all photos from RGB to CMYK, even though that might not be the best course of action.
The safest thing to do, in my opinion, is to ask your commercial printing supplier. Different printers may/will probably approach this question differently based on their equipment and expertise. Most, however, will probably ask you to leave the photos in RGB format within an InDesign file. Of course, to be safe you can always provide both a press-ready PDF file and a native InDesign file when you upload your final art. This way, your printer can always go back to the native file to make any changes he needs to make that might not be possible in a PDF file (which is essentially locked down and not as easily modified).
Another Benefit of an RGB Workflow
If you keep your images in RGB format in Photoshop and even in InDesign, you will have the added benefit of being able to easily use the documents online. And, in addition, you may have more success (i.e., rich coloration) when you print to a digital press (let’s say an HP Indigo electrophotographic—laser—press or a large-format inkjet press). Just another reason to consider an RGB image workflow.
When Not to Submit RGB Art to Your Printer
If you’re creating vector art (line art) in Illustrator, use CMYK. InDesign can convert RGB to CMYK in this case, too, but the results are not as predictable for vector images as for pixel-based images. This is especially important to note if your design includes specific logo colors (perhaps CMYK builds to match PMS colors in your logo). In this case you’ll want to have total control over the percentages of cyan, magenta, yellow, and black ink your print provider will use.
The same is true if you are applying color to selected images within an InDesign art file. Use CMYK percentages, not RGB percentages, for the reason noted above.
But don’t forget that if you’re doing anything for computer-screen reproduction, you actually want the final output to be in RGB format, and as an added benefit you get the much wider color gamut noted above.
Final Notes
If you decide to change the images yourself from RGB to CMYK prior to handing off art files to your print provider, ask your vendor for the specific color profile that will take into account the paper you’ve chosen as well as the specific custom printing device he’ll be using to produce the job. Then, in Photoshop, choose Edit/Convert to Profile (instead of just “convert from RGB to CMYK”). In this way you’ll be choosing the most precise and most accurate target specifications for your particular job, and you’ll be less likely to inadvertently omit any colors as “out of color gamut” that might have otherwise printed just fine.
This entry was posted
on Sunday, October 10th, 2021 at 1:07 pm and is filed under Photos.
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