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Converting from Colour to Black and White

Once you have taken an image and decided it will work betters as a Black and White how do you convert it. Photoshop gives you a multitude and ways but which is the best?

Switch to Greyscale

The simplest route - this removes the colour information, leaving you with an 8-bit single channel image. Files are small, but many experts feel image quality is compromised. Pictures do seem a fraction less punchy.

Image > Mode > GreyScale


Image > Adjustments > Hue/Saturation

This removes the colour but leaves the three colour channels intact. It's reckoned to produce higher image quality, although all three channels are reduced to identical 8-bit greyscale images.



Image > Mode > Labcolor

Switch to Labcolor mode then delete the 'a' and 'b' channels, leaving only the lightness channel. Again this method seems to produce crisper, punchier images than a straightforward greyscale conversion.

Gradient Map

Image > Adjustments > Gradient Map

This tool map a gradient of your choice (selected the default black-to-white gradient) against the tones in your colour image. One of Photoshop's more arcane options, but it does seem to produce well-separated tones.

Channel Changes

Image > Adjustments > Channel Mixer

Photoshop's Channel Mixer is my favourite tool for converting colour images to mono - it enables you to duplicate the effect of traditional black-and-white filters. If you check the Monochrome box at the bottom of the dialog, you can adjust the strength of the red, green and blue channels using sliders and preview the effect as you do so. Try and keep the combined percentages of the three channels close to 100 per cent to preserve overall brightness of the image. This works well for scanned images but not so well for images captured using CCDs due to the way they work.