Unsharp Masking Tips

by Darren Collins
Tuesday, 26 February 2002

Almost all digital photographs are slightly soft straight from the camera. The last step in preparing a photo for display, whether in print form or on the computer screen, should usually be to sharpen them. All photo editing applications have a Sharpen command, but it's usually a bit heavy-handed and doesn't offer you any fine control. A much better tool is the Unsharp Mask - if your application doesn't provide this, it's probably worth upgrading.

Note that sharpening should always be the last step in your process. Colour correction, cropping, resizing, etc should all be done first.

Choosing Appropriate Values

Unsharp mask is a little bit more complicated to use than the basic sharpen command. It requires three values: Amount, Radius and Threshold. Here's the technique I use to arrive at an effective set of values:

  1. Open the image you're working on in your photo editor, and set it at 100% zoom. Start the Unsharp Mask tool.
  2. Enter 500% in the Amount box. This is the maximum value, and allows you to see what the other two settings are doing to your image.
  3. Set Radius to 1.0 pixels, and Threshold to 0. The picture will look strange, but such an aggressive setting lets you see if you are enhancing noise or other unwanted detail in the image. If the picture looks too grainy or noisy, you need to adjust the Threshold setting higher (try 2, then 5, then 10 to see how the image changes). You want Threshold to be as low as possible without introducing unwanted noise.
  4. Increase the Radius value until just before it makes the picture lose detail. It should emphasise edges, but not overpower the finer details.
  5. Now adjust Amount down until the picture looks normal again. You don't want to see bright halos around dark objects - the emphasis should be subtle and look natural.




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