While reading the October 2005 edition of Photography Monthly I came across a great technique for sharpening images. Most people use unsharp mask which is generally accepted as the best way of sharpening images because of the degree of control that is available. Sometimes coloured artifacts can be introduced as a matter of course. The way to combat this is to sharpen only the ‘lightness’ channel. To do this open your image:
- Select Image->Mode->Lab Color
- Select Window->Channels
- Click on the ‘Lightness’ channel
Now perform your unsharp filter as usual. Useful starting parameters are:
Radius
300dpi print:1.5
96dpi web image:0.5
Amount
Between:100 and 150
Threshold
Start at 0 (increase this value if you find that large areas such as the sky are being sharpened too much)