I hope this is somewhat of a reasonably definitive guide to B&W conversion. There are just so many ways of converting a colour image to B&W, each one having it's own advantages and disadvantages. One method may favour one style of shot, another favour a different type. The point is, use what works for you. There is no right way or wrong, best way or crap way and anyone that tries to convince you is completely wrong.
Now in saying that, some methods are possibly more flexible than others, so people may believe they're better and more powerful. Others may argue that because they're more flexible, they're also more inconsistent and as such they're not as good. At the end of the day, you need to decide what works for you and the particular image you're working on. Don't let any one person tell you that their method is better. Experiment for yourself, make up your own mind. Definitely don't believe the hype!
Rather than detailing the "how to" of each method, I've displayed a diagram below of the before and after of each conversion method, as well as including a histogram. The before shot, shows 3 pure colours of Red, Green and Blue along the top and then a colour picture below it, containing a combination of these colours. The after shot, is that colour shot, converted using the method discussed. The histogram is included mainly to show and prove differences between techniques.
Regardless of how we get there, each method produces an image which contains only B&W tones ranging from white through to black. Whilst it's possible to tone monochrome prints, for simplicity of discussion, I haven't included it here. Also, if you're interested, I explain a bit of the maths of B&W conversions in another tutorial, but for now I'd like to start off easy. So let's begin..
Desaturate
Desaturating the colours is probably one of the simplest ways of converting a colour image to B&W. The myth about desaturate is that it throws away colour information. Whilst this is true, SO DOES EVERY OTHER CONVERSION METHOD, after all, we want to create a monochrome image. However the problem with desaturating an image is that it maps all colour tones to the same greyscale value, often leading to a very flat image. If you look at the diagram below, note how the red, green and blue elements all become the same tone, i.e. 33% red, 33% green and 33% blue.
Grayscale
Converting an image to grayscale is another very quick and easy method to create a monochrome image. If we look at the output converted image, it can be clearly seen that the red, green and blue channels all get mapped to different tones, leading to a different black and white image. THIS DOES NOT MAKE IT BETTER OR WORSE... JUST DIFFERENT. Certainly, I'd prefer to use gray scale than desaturate in general, but if my original colour image comprised mainly of one dominant colour, then the output converted image would be more or less the same regardless. Converting an image to gray scale, once again throws away all colour information. Generally when we convert a colour picture to gray scale, we take 30% of the red value, 59% of the green and 11% of the blue, to combine to give us the final output. The values are chosen due to the different relative sensitivity of the human eye to each of the primary colours. Photoshop does this automatically, but try entering these values into Channel Mixer and see for yourself.
Luminosity/Lightness
Most people think of each colour as a combination of red, green and blue. This is an RGB colour mode. Another common colour mode is CMYK which is commonly used by printers (Cyan, Magenta, Yellow, Key/Black). Another yet again is LAB (Lightness/Lumiosity, A and B). The colour is split into the A & B channels where Lightness contains the luminosity/brightness of the pixels. Converting via LAB mode, uses this luminosity mode as the base of the B&W conversion. It is colour agnostic in that the colour information is ignored for the conversion and only the relative brightness of the pixel is considered. Unlike the previous two modes, this does not have a channel mix equivalent, because the colour information is not considered before the conversion. So there is a VERY distinct different between it and converting via grey scale, as converting via grey scale definitely favours one colour over another. The differences are subtle for the below example image, but can be seen in the histogram between both images. Similarly an image with a huge cast of one colour over another will convert differently depending on what method is used. When people shoot B&W film, assuming no filter is used, the film is colour agnostic, so converting via this method should give you film-realistic results.
Channel Mixer
Channel mixer is a wonderfully powerful and flexible way of converting colour images to B&W. This is because we as the user can control the mix of Red, Green and Blue values, which make up the final gray scale image. If we use 100% red, then red will become 100% white and the green and blue components turn completely black.
If we choose 100% green, then the green element goes to pure white and both red and blue turn 100% black.
And similarly, if we choose 100% blue, then the blue element goes to pure white and both red and green turn 100% black.
The beauty of channel mixer, is that we can change the mix of channels to achieve a different B&W conversion. This is usually achieved as an adjustment layer, so the original colour information is untouched. However this does not mean we are not throwing away colour, because we are! All we are doing is controlling how much each of the individual 3 channels contribute to the final gray scale brightness. If we changed the R, G, B values to 33, 33 and 33, we'd effectively have a desaturated image. If we change them to 30, 59 and 11, we'd effectively have grey scale.
Gradient map and others
Without going into the explanation of how these methods work, there are numerous other ways to convert a colour image to gray scale. One such method is via a gradient map, another would be via calculations. Regardless of how they work, you still end up with a B&W image.
Summary
So each method has it's uses, it's advantages and disadvantages. Some favour channel mixer for it's flexibility in being able to change the mix of colours lending to the final conversion and it's non destructive nature. Some favour LAB for it's "film like" result and consistent approach in that it is colour agnostic. Other's favour gray scale or desaturate for their pure ease of use. The point is find what's right for you. Or better still, find what is right for your current image. Don't believe anyone telling you one method is crap or one is better than another.. it's simply not true. The best thing you can do when you hear someone say this, is ask them to prove it.
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