Photo Printing, RGB vs CMYK

"Colours Ain't Colours Sol"

RGB is Red Green Blue

The RGB colour model is based on the Young–Helmholtz theory of trichromatic colour vision, developed by Thomas Young and Hermann Helmholtz in the early to mid nineteenth century, and on James Clerk Maxwell's color triangle that elaborated that theory (circa 1860).

The colour TV pioneer John Logie Baird demonstrated the world's first RGB colour transmission in 1928, and also the world's first colour broadcast in 1938, in London. In his experiments, scanning and display were done mechanically by spinning colourized wheels.

CMYK is Cyan Magenta Yellow and blacK

In 1906, the Eagle Printing Ink Company incorporated the four-colour wet process inks for the first time. These four colours were cyan, magenta, yellow, and black (also known as key), hence the name CMYK. It was discovered that these four colours can be combined to produce an almost unlimited number of richer, darker tones.

RGB or CMYK?

The colour systems used by scientists and artists are entirely different. An artist will mix blue and yellow paint to get a shade of green; a scientist will mix green and red light to create yellow. The printed page in a magazine is yet another system.

It's important to define the two different kinds of colour that we see in the world as the first step in understanding colour systems:

• First, there's the colour you can touch, such as the skin of an apple or a painted wall. These colours are part of the surface of an object. (CMYK)
• Next, there's the colour you can't touch, such as a beam of red light and the colours produced by your computer monitor. (RGB)

Colours generated by light are part of one colour system (RGB). The tangible colours which are on the surface of objects or on the printed page are another colour system (CMYK).

Src: http://www.colormatters.com/color-and-design/color-systems-rgb-and-cmyk

A Practical Example

Original Photo 16 Mpixels (left)

Converted to TIFF, CMYK, poster size 34 Mpixels (middle)

Saved as smaller JPEG for displaying online or emailing but NOT Printing! (Right)

Photoshop details:

The Image / Mode menu change RGB to CMYK

The Print Preview for CMYK (left) and RGB (right)

Size is Important! But so is Quality i.e. Pixels

Print size	Image size
4x6 inches	1200 x 1800 pixels
5x7 inches	1500 x 2100 pixels
8x8 inches	2400 x 2400 pixels
8x10 inches	2400 x 3000 pixels
8.5x11 inches	2550 x 3300 pixels
9x16 inches	2700 x 4800 pixels
11x14 inches	3300 x 4200 pixels
11x16 inches	3300 x 4800 pixels
Office Works
Size	Price
4 x 6" (10 x 15cm) Print	10¢
5 x 7" (12 x 17cm) Print	30¢
6 x 8" (15 x 20cm) Print	70¢
8 x 10" (20 x 25cm) Print	$2.50
8 x 12" (20 x 30cm) Print	$3.50
11 x 14" (28 x 35 cm) Print	$13.85
12 x 16" (30 x 40cm) Print	$15.85
16 x 20" (40 x 50cm) Print	$22.85
20 x 24" (50 x 60cm) Print	$36.85
20 x 30" (50 x 75cm) Print	$42.85

Digital camera resolution vs print quality
Capture Resolution	Video Display	Print Size
		5x7"	8x10"	11x14"	16x20"	20x30"
320x240	OK	Crap	Crap	Crap	Crap	Crap
640x480 0.3Megapixel	Good	Crap	Crap	Crap	Crap	Crap
800x600	Excellent	Reasonable	Crap	Crap	Crap	Crap
1024x768	Excellent	Good	OK	Crap	Crap	Crap
1280x960 1 MP	Excellent	Very Good	Good	Crap	Crap	Crap
1536x1180	Excellent	Excellent	Very Good	OK	Crap	Crap
1600x1200 2MP	Excellent	Photo Quality	Very Good	OK	OK	Crap
2048x1536 3 Megapixel	Excellent	Photo Quality	Excellent	Good	OK	OK
2240x1680 4 Megapixel	Excellent	Photo Quality	Photo Quality	Very Good	Good	OK
2560x1920 5 Megapixel	Excellent	Photo Quality	Photo Quality	Excellent	Very Good	Very Good
3032x2008 6 Megapixel	Excellent	Photo Quality	Photo Quality	Photo Quality	Excellent	Very Good
3072x2304 7 Megapixel	Excellent	Photo Quality	Photo Quality	Photo Quality	Excellent	Excellent
3264x2448 8 Megapixel	Excellent	Photo Quality	Photo Quality	Photo Quality	Photo Quality	Excellent
10 Megapixel +	Excellent	Photo Quality	Photo Quality	Photo Quality	Photo Quality	Photo Quality

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