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Photoshop Tutorials For Beginners|Lesson 10: Why Colour Modes and Bit Depth Matter
One of the most important things for using a colour managed worflow is to understand the difference between RGB and CMYK colour modes (or 'models'). So that's what I'll be explaining in this lesson. We'll also look at the subject of bit depth, which is really simple but causes lots of confusion. So let's get these things cleared up...
The difference between CMYK and RGB
RGB and CMYK are 2 different colour modes. The differences between them are very simple:
RGB: This colour mode is used in all devices that use light to represent colours. For example, our digital cameras use RGB colour mode, as do TV's and computer screens. The letters stand for red, green and blue, which are referred to as 'colour channels'. When red, green and blue are mixed together they make white.
Adjusted red, green and blue colur channels on a Curves adjustment layer:
There are different RGB options, known as colour spaces. These are: sRGB, Adobe RGB (1998) and ProPhoto RGB.
You should select which RGB colour space to use in your camera menus, and then match that colour space in Photoshop, by going to Edit>Colour Settings. If you shoot in Raw, images will not be 'encoded' into a particular colour space at all in your camera, so you just have to select one to use in Photoshop.
ProPhoto RGB produces the largest colour gamut, then Adobe RGB, then sRGB. Most people choose to shoot (and therefore edit) in Adobe RGB. There are lots of factors for deciding which colour space to use, but I personally work in ProPhoto RGB because of the large colour gamut.
Always make sure that Photoshop's Color Settings match the RGB space your camera is set to. A good way of ensuring this is to select 'Preserve Embedded Profiles' from the RGB option in the 'Color Management Policies' section of the 'Color Settings' dialogue box.
CMYK: This colour mode represents colours by mixing together inks, and is used by digital photo printers. The letters stand for cyan, magenta and yellow, with 'k' signifying the 'key' colour of black. Black is the colour that results when cyan, magenta and yellow are mixed together.
Cyan, magenta, yellow and black:
A really important thing to understand is that the CMYK colour mode, used by our printers, is not capable of representing the same colour gamut (range) as the RGB mode on our cameras and monitors (CMYK technically has the same number of colours, but cannot render them with the same range of saturation, which makes a big difference). So CMYK pictures (prints) are often more faded and dull than RGB pictures.
Previewing CMYK prints in Photoshop
Whilst working on an RGB image in Photoshop, we can easily check how the colours we see will translate into a CMYK colour space. Go to the View menu and choose Proof Colours.
This switches the display of your selected image to something akin to a CMYK print. In the file name tab you'll see "/CMYK" appear. A shortcut for this is to hit Cmd/Ctrl + Y - a really handy one to know!
You can be even more precise in previewing a CMYK version, by selecting the exact profile you will be printing with. Go to View>Proof Setup>Cutom and then choose a profile from the "Device to Simulate" list. (It's crucial that you do work with profiles when printing, and they can be easily downloaded from your printer manufacturer's website).
Another really interesting way to illustrate the difference between RGB and CMYK (specifically the limitations of CMYK) is to click on the foreground/background colour squares in the toolbar. When the colour picker appears, select a point in the absolute top right corner: a really zingy, vibrant red.
As you do that, a warning exclamation mark will appear. This means that the selected colour cannot be rendered in CMYK (or, technically, the selected level of saturation. CMYK can of course produce the colour red). It cannot be printed. Clicking on the exclamation mark will switch to the nearest colour that can be printed in CMYK. It's a surprisingly big difference and illustrates the gap between RGB and CMYK.
Just what is "bit depth"?
Bit depth sounds horribly technical, doesn't it? But it's just the term used to describe the potential colour gamut within an image. The greater the bit depth, the more control you have in editing and fine tuning colours in Photoshop.
8 bit: This is basically a typical colour JPEG image, in which there are 256 versions of each of the 3 colour channels (256 reds, 256 greens and 256 blues). This means that each of those colours can be combined in different ways, giving 256 x 256 x 256 possible colours, which comes to 16.8 million.
(By the way, 1 bit = 2. You can work out the number of colours per bit by placing the number of bits to the power of 2. So 8 bits is 2 to the power of 8, or 2x2x2x2x2x2x2x2, which is 256).
16 bit: This is the bit depth available when shooting in Raw. There are 65,536 versions of each of the 3 main colour channels. 65,536 reds x 65,536 greens x 65,536 blues produces a total colour range 281 trillion different colours.
It's always worth shooting in Raw to give you the added flexibility of editing in 16 bit. To work in 16 bit mode, open your image and go to Image>Mode and tick 16 Bits/Channel. When major edits are made to images in 8 bit mode, 'banding' can occur. This is where horizontal or vertical lines appear in areas of high contrast, ruining photos. Working in 16 bit reduces the the chances of banding a great deal.
Banding, which would be avoided if this image were edited in 16 bit mode:
So that's lesson 2 on Photoshop technical essentials done! RGB and CMYK colour modes are very different, but you can easily compare your RGB photo to the CMYK version that will be printed in Photoshop. With a colour managed workflow and a good quality printer you can get some really good, accurate prints. Shooting in Raw gives you the greatest control over colours in Photoshop.