Colour to monochrome conversion techniques using Photoshop

So what's the best way to convert your colour images to monochrome (black and white)? There are many methods, but here are four of the simplest and most important ones. The interesting thing is that each gives a subtly different result. You may want to try out a couple of them on any given photo before you make a final choice.

Three of these conversion processes are automatic, in that you have no control over the tonal relationships. The fourth, using the channel mixer, is the most complex, but also the most interesting.

Convert to Greyscale


(Image > mode > Greyscale). This is the simplest method for converting colour images to black and white, and maybe for this reason it's not generally regarded very highly amongst black & white fans. However, it actually does a petty good job separating colours tonally - more so than simply 'desaturating' the image. What's more it reduces the number of colour channels in the image from three (RGB) to one (greyscale) so your files are now one-third the size. The disadvantage is that you will then have to convert the image back to RGB (it will still be grey since the colour has been removed) in order to apply many Photoshop filters and controls.

Desaturate the colours


You can retain the three colour channels in your image by desaturating it (Image > Adjustments > Desaturate). This saves you having to convert to greyscale and back to RGB for adjustments that only work on RGB images. However, although desaturation sounds ideal in principle, in that you're eliminating colour without altering the brightness values, it can leave colours looking tonally very similar.

Use Lab Mode

Lab mode

Many mono experts prefer this slightly more complex solution. If you switch to LAB mode (Image > Mode > Lab Color), Photoshop splits the image information in a very different way. Instead of Red, Green and Blue channels, there's now a lightness channel accompanied by two ('a' and 'b') colour channels. If you delete both the 'a' and 'b' channels then convert the image into greyscale mode, you have in theory ditched all the colour information and retained only the lightness values.

Shouldn't this give the same results as desaturation? It doesn't. It separates colours just as well as the greyscale conversion, but also gives noticeably lighter, more open tones.

Channel Mixer

Channel Mixer

The Channel Mixer (image > adjustments > channel mixer) is the most complex conversion option but can also be the most useful. First, make sure the Monochrome box is checked. Now you can adjust the proportion of the red, green and blue channels to mimic the effect of traditional black and white filters. Using only the red channel will have the same effect as using a red filter with monochrome film. Non-red colours (like the blue sky) are darkened, while red colours are lightened. When the process is complete you have an RGB image that is perfect for toning and further editing.

Written by Phil Calam

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