sRGB vs. RGB Locked

Started Apr 22, 2013 | Discussions thread
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John King
John King Forum Pro • Posts: 14,941
Shoot RAW, and use aRGB - a colour tutorial

Gidday GM

GMartin wrote:

So is RGB a color space or NOT ?

RGB (Red, Green, Blue) is the generic description for all transmissive colour spaces - i.e. ones where the image (etc) is illuminated from behind. It is an additive colour system, in that if you have full values for each of the Red, Green and Blue channel (each has a value of 255 in an 8 bit colour space), then the light you see will be white. If all three values are 0, you will see black.

RGB is the "native" colour of light until it bounces off something. Then it becomes a subtractive colour space. We see a flower as red because most of the blue and green parts of the spectrum have been absorbed, leaving the red part to be reflected.

CMYK is generic description for all reflective colour spaces. These are always subtractive colour spaces. Unlike RGB, when one mixes Cyan, Magenta and Yellow (CMY) in equal proportions, one does not get black. One gets a muddy, dark brown. Hence the addition of pure black (K) in printing so that blacks can be properly reproduced.

All modern printing processes since around 1500 AD have used the four colour printing process, CMYK, My Epson R3880 uses an 8 colour printing process, using three different blacks and cyan, light cyan, magenta, light magenta, and yellow. This allows for the printing of colours that are way out of gamut for sRGB and even aRGB.

sRGB is a deficient colour space in that it can only cover a small range of colours (gamut). It is also a defective colour space in that the three colour axes, R, G and B, are not equal in length. sRGB is particularly defective in the greens, and somewhat less so in the reds. The small gamut impacts on the ability to reproduce saturated colours; the defect impacts on its ability to reproduce colours accurately, relative to both reality and each other. Using sRGB causes bright red cars to reproduce as a sickly pinkish orange ...

aRGB is a much wider colour space than sRGB. It is also not defective, in that the three axes are approximately equal. There are some colours that aRGB cannot reproduce well, such as the saturated yellows and purples in flowers and many other things. aRGB is a "safe" colour space for editing, your own viewing and printing.

The bit depth of a colour space determines the absolute number of colour numbers available to it. A 16 bit colour space allows for far more colour numbers than an 8 bit colour space, and is far more suitable for editing IME.

Personally, I use ProPhotoRGB, 16 bit for editing, even though my monitor can only display a 10 bit aRGB colour space. IOW, I cannot see what I am doing ... However, editing in this way allows much more delicacy in the final print, with greater colour nuance.

My camera/s can capture a wider gamut than an aRGB colour space, at a higher bit depth than 8 bit.
My monitor can display most of the colours captured.
My printer can print the gamut captured by the camera shooting RAW
(+simultaneous large JPEGs).

If my 10 y.o. laptop can manage to run PS5, I am certain that your desktop can do likewise ...

My advice?

Shoot RAW, or at least aRGB. If you must shoot JPEGs only, use aRGB colour space for everything except posting to the web.

Buy Photoshop Elements - it's cheap and powerful, and won't overtax your computer (or your wallet).

Calibrate your monitor (actually your graphics card ... ) so that you have some idea of what you are doing. A Spyder 4 Express calibrator costs less than $100.

The ONLY time I use sRGB is for the web and sending other people images by email etc.

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Regards, john from Melbourne, Australia.
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