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I seems that all cameras take photos in RGB format and printers take photos in CMYK format. If the photo is not in CMYK, then the printer converts it to that format with some loss of quality. Can the loss in quality be noticeably bad ? Is it possible to set my camera, say Canon T3i or Nikon D3100 to take photos in CMYK mode or something similar ? Or is it better to convert the image using some software ?

Thanks.

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You might want to read up on these: en.wikipedia.org/wiki/Subtractive_color and en.wikipedia.org/wiki/Additive_color –  Szabolcs Mar 23 at 20:28
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This, of course, assumes that the printer is CMYK, and that's only true in 4 colour trade press production and at the "low end" (with the acknowledgement that "low end" can include some fairly hefty price tags due to production rate and media handling). It's not true of hexachrome and other high-end multicolour press processes, and it's certainly not true of photographic/"light jet" printers or high-end inkjets. And which CMYK? There are a lot of them due to various ink types, with only the K (black) being more-or-less stable across them (and even then, black is not neutral). –  user2719 Mar 24 at 3:43

4 Answers 4

up vote 11 down vote accepted

Inherently, no. The RGB model is natural for recording light, and the CMYK model is natural for printing (where reflected light is subtracted). But see Are RGB numeric values equal to CMYK percentages? — the loss in conversation isn't inherently because RGB to CMYK is inherently lossy, but because the actual color spaces of the devices used are different, and converting in camera wouldn't do anything to help with that.

Some early digital cameras experimented with CMY filters (often CMYG). But these weren't actually producing CMYK output; it still went to RGB. The idea was basically that CMY primaries allow lighter, more transparent filters and therefore more sensitivity in low-light - but in practice, the conversion involves subtracting channels that don't share information, raising the noise and canceling out any advantage. Plus, there were apparently problems getting the filter dyes to align nicely. So, those have pretty much gone by the wayside.

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Well, as pointed out to me, the inherently part of that isn't actually true. There were cameras with CMYK based sensors, they just didn't stick around. Saw one reasonable explanation for it, but there's no technical reason that they can't exist and they clearly did. –  John Cavan Mar 24 at 0:50
    
@JohnCavan I'll update this with some more nuance, although probably not for a few days as I have a busy week ahead. –  mattdm Mar 24 at 11:52
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@JohnCavan Er? Are you sure about that? I've heard of CYMG based cameras, but never CMYK. –  Monk Mar 24 at 12:33
    
@Monk - Not CMYK specifically (see my answer), just noting that the model is not inherently unavailable for sensors. –  John Cavan Mar 24 at 12:57
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I'd like to see a sensor with a black filter! –  Matt Grum Mar 25 at 12:38

You have to make a distinction between mixing lights and mixing ink. When you take a picture, whatever camera you have (Digital, Analog, iPad, iPhone...) you are capturing lights.

And according to physics laws, in the lights, the white is the mix of all other colors. The rainbow is a good example of lights diffraction. It's simply a separation of the white light to the different colors. And if you analyse the rainbow, you can notice that all the colors can be obtained using a gradient of Red, Green and Blue with different intensities. So when you capture a white photo and display it in your monitor, your are mixing the Red, The Green and the Blue "sub pixels" of your monitor at their maximum intensities...

However when we talk about printing, it's another story. The printing support is already white (the paper) so logically, whenever the printer finds a white space in the picture it should print nothing on this area. With this example we start differentiating between the RGB mode and CMYK mode.

Now let's continue further. If you take oil painting and you mix Red, Green and Blue. Will you get white ? No ! It's gonna be probably something close to brown. Not even black. So to have "all" the color possibilities you need to mix Cyan (which is close to blue), Magenta (Close to Red), Yellow and Black.

As for the Cyan and the Magenta: these are simply equivalents to the RGB Mode. The black is there to darken the black spaces in a picture but there is no green like the RGB mode. Why? The green it self is a combination of the Blue and the Yellow in the physical world. Try mixing oil painting and you will see. So this is why in printing, you need CMYK colors which are the most basic physical color substances.

Now the conversion can be done using software such as Photoshop.

Hope it clarifies.

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Cameras are RGB because of the color filters over each of the pixels (photosites) on the sensor. There are a couple of rare cameras with a filter that instead samples cyan, magenta and yellow (not sure about luminance (black)), however this then would require a conversion to RGB to work with most screens and even a lot of photo printers which are RGB based.

That said, conversion from RGB to CMYK doesn't really have significant problems. There are some parts of the color space that can't overlap particularly well, but it isn't a major detractor.

Also, not all printing is CMYK. In fact, CMYK is primarily used for document and mass publication printing. C-type photo printers (which directly expose photo paper to print a photo) generally use either RGB lasers or RGB LEDs to expose the paper in true RGB.

Similarly, ink jet photo printers use neither CMYK or RGB, but rather a combination of numerous different colors (my printer uses 12 different colors) to achieve the desired final color.

Thus, it doesn't particularly matter what system the camera works in and since monitors and TVs work in the RGB color space and it requires less complexity to process, it makes sense to have cameras work in the RGB color space and then convert color spaces only when necessary.

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No theory to that, Nikon and Kodak (at least) did produce CMY cameras and they were commercially available. I was a bit surprised to dig that out. –  John Cavan Mar 24 at 1:01
    
@JohnCavan - ah, glad you found that. I had thought there were too, but I couldn't find details when I went digging. –  AJ Henderson Mar 24 at 1:17
    
Can't take credit, it got pointed out as a comment to my answer, so I went for a look. –  John Cavan Mar 24 at 1:19

So, the vast majority of sensors are RGB (the array is usually 1 red, 1 blue, and 2 greens or Foveon), so right off the bat you're working with handicap with respect to CMYK. However, that's not really the reason why this doesn't work...

CMYK is a subtractive color model, it works by subtracting from white which is the presence of all colors. RGB is additive, it works by adding the colors to make white. If you consider the sensor a bit, it basically works by gathering light into photoreceptors and that's inherently additive process, it adds some amounts of lights from the red, green, and blue parts of the spectrum to produce the final image.

Doing a little bit more digging (thanks @Gabe for the nudge)... If you drop K (key, which is black), then you could do this. Nikon and Kodak, at least, had CMY(GY) cameras with green or yellow as the fourth color. I haven't found a great explanation for the G, but the premise of two green in the Bayer model is true, then a green or yellow additional makes sense since we're more sensitive to it.

So, I guess the question would be... why did GRBG (or variants) win the race as it were? According to Olympus Micro:

The downside of CMY filters is a more complex color correction matrix required to convert CMY data collected from the sensor into RGB values that are necessary in order to print or display images on a computer monitor.

They have a pretty nice article on this: Introduction to CMOS Image Sensors and the explanation makes sense. If the vast majority of photographs taken are going to be displayed on a monitor, then GRGB sensors make it easier.

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A typical sensor is not inherently RGB (aside from those like the FOVEON X3). It is the Bayer pattern that makes it RGB, and not all cameras have GRGB mosaic filters. If you read dpreview.com/reviews/kodakdcs620x, you'll see that 15 years ago it was not uncommon to have CYGM or CYMY filters on cameras. –  Gabe Mar 23 at 20:11
    
They are since they have the filters in place. There are exceptions, of course, but the vast majority are based on the traditional bayer model. Nevertheless, I can tweak the answer to be more clear. –  John Cavan Mar 23 at 20:17
    
Would not another downside be that if a CMY sensor is calibrated so that white is 1,1,1, then a fully-saturated cyan, magenta, or yellow subject would only have one f-stop difference between different color sensor readings (whereas RGB would have one sensor read black for any fully-saturated color). –  supercat Mar 23 at 22:51
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Did the CMY sensors actually perceive more of the CMYK gamut than an RGB sensor? If so, that would be a pretty significant advantage over RGB. –  drfrogsplat Mar 24 at 0:44
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@drfrogsplat No the CMKY printable gamut is totally different to the CMY sensor gamut. CMY sensors collect more light since you're only filtering out 1/3 of the incoming light at each pixel, compared to filtering out 2/3rds of the light with RGB (the yellow filter in CMY only filters out blue light, whereas the red filter in RGB filters out blue and green light). The downside is that you have to infer the RGB values by comparing neighboring pixels (which you do to an extent with RGB) meaning poorer colour separation. –  Matt Grum Mar 25 at 12:37

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