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When you send photographs from your camera to the photo-lab to get some prints, it is OK to send them as JPEG files with the RGB color system that the camera assigns to them by default.

When you send a design, lets say for a brochure or a flyer, to a print shop, they specifically ask for CMYK. Why is it different for photo labs?

The designs for brochures, flyers, business cards, etc also contain pictures often. Is the process different for photo prints? Does the photo-lab makes the conversion from RGB to CMYK for you, while print-shops just want you to do it yourself? What I still do not understand is: For a flyer, If the printshop is asking for CMYK, then i will convert my pictures to CMYK. But in the conversion from RGB to CMYK there will be some colors lost or not accurately matched. So no matter if I send my picture files in CMYK, My pics are doomed to lose their original look. Or is there a way to convert from a JPG RGB (which is what my camera produces) to CMYK without the risk to get pics coming out dark or dull or just different than the original?

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migrated from graphicdesign.stackexchange.com Nov 22 '13 at 15:00

This question came from our site for Graphic Design professionals, students, and enthusiasts.

    
Just wanted to comment that JPEG is a lossy compression algorithm, which means you are losing color data when you save as a JPEG. Most of this color data is beyond our ability to detect it, which is why it's lost. But the algorithm requires converting the color mode from RGB to YCbCr which is based on brightness and chroma of colors. –  OghmaOsiris Nov 22 '13 at 15:36

4 Answers 4

Lab employee here. I can't speak for all labs but here's how it works at mine (note that we do both photographic and press printing as well as a few other methods).

It all depends on the type of equipment that is going to be printing your product. Traditionally photographic prints are done on a minilab or equivalent piece of equipment which prints in RGB. When you order a flyer or a business card it is going to be done using a press which prints in CYMK. There's many other ways of printing beyond photographic and press but those will be the most common.

While it is true that one can convert between the two color spaces there's certain colors that simply can't be converted. Take a look at the following image for an idea of the different gamuts that exist for various color spaces:

enter image description here

If you were to send an image in RGB to be printed on a CYMK device where there's colors that fall out of the gamut of what CYMK can achieve, a substitution will occur that attempts to get close to your intended color. This can cause your once vibrant and beautiful RGB image to come out looking dark and dull on a CYMK printer for example.

For this reason we suggest customers send us their images in RGB format when a photographic product will be produced and CYMK when ordering a press printed product. In reality most of our press printed work is also sent to us in RGB and our customers feel the output is satisfactory. Not all customers feel this way and those that want truly accurate color do send their press printed images in CYMK color space.

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2  
I would just point out that these days, with advanced printers that use advanced inks like Epson UltraChrome or Canon Lucia, the print gamut is close to, and in some cases even larger than, sRGB (which would roughly conform to your "monitor" profile in the posted image.) Additionally, while you cannot "preserve" exact color information during a conversion, you can still convert colors that are out of gamut by compressing the larger gamut into the smaller gamut. –  jrista Nov 23 '13 at 8:54
    
"In some cases" is putting it mildly. Even going back a couple generations, UltraChrome K3 and the original LUCIA were meaningfully breaking out of Adobe RGB and into ProPhoto RGB in multiple areas. This does naturally vary with the media (matte papers will have a smaller gamut, uncoated papers a smaller one still), but gamut's been exceeding sRGB for quite some time. –  colinm Jan 10 at 0:04

It depends on the lab. Some processes are designed for CMYK others for RGB. Most labs can also adapt the format, though if you want the finest grain control, you should proof in whatever color space they will use. This is also why they ask for them in the correct format. They don't want to have to deal with unhappy customers as a result of the change in color space.

There are chemical photo paper labs that use RGB LED and laser based exposure of the photo paper. Non-photo print shops tend to use a CMYK screening process because it is faster to mass print. Even when they aren't using screen prints, it is the ink colors they work with and the color system that graphic designers are used to for their spot color selection.

Photo on the other hand uses RGB cameras primarily and works in the RGB color space, so RGB makes more sense as a final output color space and thus the printers follow suit. This is to say nothing of inkjet and pigment systems that use numerous (mine uses 12) colors in order to fill out the color space even more evenly.

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It is about accountability. Typically print jobs have company branding involved. Printers don't want clients returning large print batches because "The blue is not the same". There is less room for error (no conversion on their end) if you supply them the document in the same format they are printing it in.

Slight color variations are also harder to spot in a photo than in large areas of flat color.

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The CMYK colour space is based on a subtractive colour model, where combining colours results in a darker shade. This is the way that inks work, by absorbing incoming light - theoretically mixing cyan magenta and yellow together will result in black (K stands for keyline and is a black ink that is used because mixtures of CMY tend to produce slightly off black results).

RGB is an additive model - this is the way that light-sources like a computer monitor work, mixing red green and blue results in white. Photo labs tend to produce prints by tracing the image onto light sensitive paper using a laser. In this process the RGB model makes more sense as it directly translates into the light that needs to be generated. CMYK is used for bulk printing because it is faster and cheaper.

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