A number of my 24bit colour images have been converted to monochrome, and I have become increasingly aware that not all of the procedures available in Photoshop give exactly the same results. This page takes a comparative look at some of these alternative methods, starting with the full colour test strip shown above. Both sets of primary colours, subtractive (yellow, magenta, cyan) and additive (blue, green, red), have been included.

One of the simplest ways to produce a monochrome is to desaturate the image: choose Image>Adjust>Desaturate, from menu bar. This is a quick and simple method that maintains all three RGB colour channels, but it is not equivalent to greyscale mode (as sometimes suggested) for the reasons that follow. Notice that the tonal values of all six primary colours have become uniform. The image of the old lady has not fared too well either. A similar result is obtained at this stage via  the "Hue & Saturation" or "Colorize" options.

The next approach is to convert to greyscale mode: choose Image>Mode>Greyscale, from menu bar. More tonal differentiation is apparent, particularly in the primary colours, when compared to the desaturated example above. This is because the luminosity levels of the original pixels have been retained. One disadvantage of greyscale mode is that RGB channel information is lost and we are left with one 8-bit channel. It is possible - and to my mind advantageous - to convert back to RGB mode, although the original colour data is lost. 

Incidentally, if you print an 8-bit greyscale using colour, the image is automatically changed to 24-bit RGB to facilitate the use of coloured inks. Personally, I don't favour printing greyscale in black only, as the printer has to dither more in order to represent 256 shades of grey. Although this has the advantage of avoiding colour bias, the result is rather too grainy for my liking.

Those of you with Photoshop 5.(x), or later, will be able to use the Channel Mixer option: choose Image>Adjust>Channel Mixer. A dialogue box is opened, and you should check the monochrome box therein. Three % sliders (Red, Green, Blue) are available for fine tuning the tonal contribution of each colour channel. The defaults are 100% Red, 0% Green, 0% Blue. 

In the above example I used: 80% Red, 16% Green, 4% Blue, but the final mix is entirely up to you. Notice that the blues and greens here are relatively darker, leaving the skin tones a bit too light. This method offers a degree of control, whilst allowing a high quality greyscale to be achieved, but good results are not automatic, however, and this is where personal judgement comes into play. Well worth experimenting with.

This example is the same as the previous channel mix option, but I have introduced a slight sepia tone by adjusting the colour balance of the midtones and shadows, leaving the highlights untouched. Choose Image>Adjust>Color Balance, then select a tone range. Use the sliders to increase the amount of red and yellow - or the colour balance preferred.

In the past I often used "colorize" in Photoshop to produce sepia or neutral tones, before noticing the levelling effect it seems to have on tonal distribution. In this example, however, tonal differences have clearly been maintained in the primary colours. This is because the image was first converted to monochrome with Channel Mixer - as in the previous two examples - and although the original colour information has been lost, the luminocity values of the colour channels have been retained. 

A further observation concerns the way in which "colorize" has introduced a hue change across the whole tonal range. Using "Color Balance", as in the previous example, it is possible to leave the highlights to some extent unaffected, thereby retaining a greater element of white and neutral grey in the picture.

It is also possible to obtain monochrome versions of colour images by selecting and copying one of the RGB color channels that make up the image - they can be found in the Layers Palette. A similar route is to change to LAB mode and select the "lightness" channel. Having made a copy of a preferred channel, simply paste it into a new file: choose File>New and give it a title.

Faced with such an array of options can be daunting, but I don't think any single method can necessarily be relied upon to get the best results. Some certainly have more control than others, and it is important to be aware of the pitfalls, but ultimately you have to decide which result best matches your own aesthetic preferences or intentions. Hopefully, the above information will serve as a useful guide.