Wednesday, 6 August 2008

Sharpening using unsharp masks, part 2

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To set the scene I recommend you read my earlier post - Sharpening using unsharp masks.

If you work through the procedure detailed above you will create an unsharp mask of your image which, when overlayed with the original image, gives the appearance of sharpening.

Why does this work?

Two desaturated (colour information removed) copies of the image are created, one is inverted (black to white, white to black) and the blending mode (a way of combining two layers in Photoshop) between the two is switched to colour dodge. In this particular blending mode blending with black has no effect on the image contained in the other layer and blending with white gives white. Since the images are the inverse of each other the blended image is white.

Here's the clever bit. The inverse image is now blurred slightly. The new non-white pixels added due to blurring will overlap with the non-white pixels in the original and an edge outline of the original image is observed. These layers are merged to leave the outline which is then combined with the original image using the multiply blending mode. This causes the overlapping areas to become darker (result colour = top colour x bottom colour/255) and hence the contrast in the edges of the image is increased.

This increase in contrast gives the appearance of sharpness.

If you're confused feel free to post a comment. There's more on sharpening to follow.

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Anonymous said...

Hi David. I just wondered if you leave your camera's default sharpening for jpegs or camera raw sharpening defaults for raw files before you do unsharp mask? Or do you zero out any default sharpening in your camera or photoshop/lightroom first? Thanks.

David Fenwick said...


I shoot RAW which means I'm working with untreated data from the sensor (there's no such thing as camera RAW sharpening defaults).

I sharpen my images using a Photoshop droplet on exporting from Lightroom.

Obviously if you shoot JPEGs then your camera has to convert the RAW data as the image is recorded and sharpening can be applied. If you're happy with the results that this provides then it saves you time later on. But once you've done it you can't remove it!

All the best,


Anonymous said...

Thanks David for your swift response. I suppose what is confusing me is that in CS6 ACR I have a tab called 'Detail'. When i click that there are some preset sharpening settings such as Amount 25, Radius 1.0, Detail 25, Masking 0. So I presumed once I open a Raw image from ACR then these sharpening settings were applied? Although I don't really notice much difference unless I apply Unsharp Mask on the opened image. Any ideas? Thank you.

David Fenwick said...

If you render an image from your RAW file using these settings then you will be applying a very small amount of sharpening. I imagine Adobe have this as the default setting so that people don't think their images are out of focus when they zoom in to 100% (digital images have to be sharpened - they're inherently soft).

I have this turned off and sharpen using an automated USM-based action on exporting my images - it's just quicker for me.