# RGB and CMYK



## dtornabene1 (Jan 1, 2009)

A quick discussion about color channels.

There are many factors effecting the way an image looks.  Many times in this forum, color channel issues are confused for WB.  

There are more than two types of images, but to keep it simple, let's just refer to the two most common types, simple RGB and CMYK.  

Most people know RGB stands for Red/ Green, and Blue.  CMYK stands for Cyan, Magenta, Yellow, and Key (Black).  RGB works by subtraction and CMYK works by addition.

Your monitor displays RGB.  If you see white on a monitor, all colors (red, green, and blue) are turned on.  In CMYK if all colors are turned on, you will get a pure dark black. 

We primarily use RGB for images on our computer and for color (some B/W, black and white) photography.  CMYK is used for printing press operations.  This is why you can send in a photograph to a printer and get a postcard that does not actually look like the one you took.  They use different colors.

CMYK and RGB have different color Gamuts (meaning they can reproduce different colors).  So CMYK can not accurately reproduces a deep blue for example.  Remember, C stands for Cyan which is like a baby blue.

So recently I was asked what I meant by increasing the green. channel in a photograph.  Each photograph has a histogram.  This histogram is the combination of all three RGB color channels.  

You can break down the histogram into its respective color channels, also displayed as a histogram.  So, I see blue and red, but green is short.  If one where to look at the histogram for each color you can see each colors highs and lows (the point of a histogram).

The point is, we can adjust the image by more than just WB and contrast.  Look at the ingredients of a photograph.  Just like a fine meal, changing the ingredients, each RGB channel, even a little, make the outcome perfect.

If you have any questions, please feel free to ask.  I am sure I am not the only one with a prepress background.

-Nick

This is the original forum the question was asked.  By the way, dont question whether or not I am correct about the green channel in the photograph here.  Do that in the post below.

http://www.thephotoforum.com/forum/photography-beginners-forum-photo-gallery/150609-overexposed.html


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## dtornabene1 (Jan 2, 2009)

I honestly thought more people would get involved in this discussion.  I was getting so many questions about the difference between photographic and offset printing.

-Nick


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## Jedo_03 (Jan 3, 2009)

Hello
No... I wasn't asking what you meant by "increasing the green..." - not in terms of the green channel and it's histogram...
Anyways, in sRGB, aRGB, CMYK, we can only adjust hue in Photoshop: we can't adjust luminosity without altering the hue... we can apply luminosity layers, but there is no blending mode for luminosity. Nearest we can come is using L of L*a*b...
The fact that some camera histograms employ green-channel data to display a histogram for luminosity doesn't necessarily mean that adjusting the green channel in PP will 'adjust' luminosity - not in a simple way...
The reason I 'asked' you (in the other thread) was that I didn't see a problem with the green channel histogram - in terms of how much green there was actually in the image - there wouldn't be any data in the green channel of an image of Santa's Hat...
ETA - Many photographic print houses use CMYK these days - they just prefer you to submit sRGB data - you can obtain the ICC to enable you to softproof before submission - or you can soft-convert in PS in the smaller gamut CMYK: even my three desk-top printers are 4-ink CMYK... but the data I send to my printers is sRGB and my printer has the workflow ICC... 
Jedo


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## ANDS! (Jan 3, 2009)

Not to offend, but this is a pretty "confusing" explanation.  Not exactly sure whats trying to be explained here beyond the original threads topic.  A historgram (and individual color channels) most definitely are related to color casts and white  balance issues.  Off kilter white balance, under/over exposure - all of this will manifest itself in a whacked out RGB histogram.


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## Garbz (Jan 3, 2009)

I don't understand how CMYK is relevant at all to photography. Processing is mostly done in RGB and in every lab I know of the only thing that needs to be taken care of is the colour profiles. The rest is transparent to the photographer.

Now when making business cards or printing a magazine it's an entirely different ball game. But even that is not normally done by photographers.


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## ANDS! (Jan 3, 2009)

I suppose some people like to make color adjustments in that mode so they can be super duper sure - that what they produce on the screen will translate to the printer.  I've never had a problem with printing colors, that would necessitate a jump to  CMYK for any editing.


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## Ls3D (Jan 3, 2009)

Funny because I don't know a single photographer that has NOT done some pre-press, designed and ordered cards and brochures.

Perhaps things are different in Garbzland.

Felling better?  Were R U now?

-Shea


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## Garbz (Jan 3, 2009)

Fair enough professional vs amateur debate. Given that this thread was in the Beginners forum I made the assumption that 99% of the people reading this were not professionals who make business cards . I am going to Bratislava and Budapest tomorrow for a few days.


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