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Sharpening – the true story
Posted by: byRo on 12-31-1969.

"High Pass vs Unsharp Mask Sharpening is a subject that comes up quite often in the threads. There are those that defend use of the Unsharp Mask (USM) and others who say that the High-Pass method is better. Let’s look into this...... "

First we’ll define the most simple application of the two methods.


USM Sharpening


There is only one step (1) to do in basic USM sharpening

- Filter>Sharpen>Unsharp Mask... at Radius “X”, Amount “Z” and Threshold 0 (*).


“X” is often specified as a pre-defined number but, if you read this tutorial , you know that it has no fixed value but that it depends on the image you have and the result that you want.

 “Z” may be pre-defined or eye-balled to taste using the preview window. Sometimes comes with advice to apply in small steps to be “smoother” (2)

(*) Very, very rarely will you see anyone using any value different to 0. (3)


High-Pass Sharpening


There are a few steps to do in basic High-pass sharpening:

- Duplicate the layer;

- Filter>Other>High Pass ... at radius “X”;

- Set this layer’s blending to “Y”;

- Vary opacity to taste;

- Merge.(4)


“X” is often specified as a pre-defined number but, if you read this tutorial, you know that it has no fixed value but that it depends on the image you have and the result that you want.

“Y” is usually specified as Overlay, or Soft Light for less, or Hard Light for more. I only once saw Linear Light mentioned as a sort of wild colouring-outside-the-lines extreme option.(5) (6)


Testing, testing....

Try this little experiment:


- Take an image, any image;

- Duplicate the background layer– call this new layer “High Pass 2”

- Duplicate again – call this new layer “USM 2”

- On the “USM 2” layer run USM filter amount 100%, radius 2, threshold 0;

- Turn off the “USM 2” layer

- On the “HP 2” layer run High Pass, radius 2, set blending to Linear Light and Opacity 50%.

Toggle the “USM 2” layer on and off (little eye-ball on the layer).  You will now be looking alternately at the results of High-pass sharpening and USM sharpening

See the difference between USM sharpening and High-Pass sharpening. If you did this right – then you won’t!


For the skepticals – set the blending of the USM layer to Difference and look at the Histogram – I got an average level difference of 0.16 (that’s one sixth of a single pixel level)


Which means....

.....that we come to some interesting conclusions:


1)      USM sharpening is nothing more than just a specific case of High-pass sharpening.

(6) More interesting still, is that it corresponds to the little-used wild colouring-outside-the-lines extreme option! Note that this is probably because blending Linear Light at 100% opacity would be equivalent to a USM Amount of 200% - which is usually pretty “wild”.

2)      The USM method is quick and straight to the point.

(1) Using the USM there are no intermediate steps and so the sharpened image comes out as a direct result. This means that USM can easily be used to sharpen individual channels or layer masks.

3)      The High-pass method is more flexible - I.

(5) The Linear Light blending is just one of the options available. Soft Light, Overlay or Hard Light are often preferable.

4)      The High-pass method is more flexible - II.

(4) The High Pass layer acts like an adjustment layer. There is no need to merge immediately, and you may change the blending and opacity at any time later. Or even build up a stack of High Pass layers with sharpening at different radii.

5)      The High-pass method is more flexible - III.

Any time you wish you can change from sharpening do blurring just by inverting <ctrl><I> the High-pass layer.

6)      The USM has a threshold adjust.

(3) Although usually ignored, this is a very useful feature which is difficult to mimick using the high-pass method.

7)      Applying USM in small steps IS NOT smoother.

(2) OK, this isn’t shown in our little experiment above. But it is very easy to prove:

- Make two duplicate copies of an image.

- On one apply USM, amount 500%, at any reasonable radius – I used 2.

- On the other apply USM 50% at the same radius. Reapply (using <ctrl><F>) 9 times.


I think it’s pretty plain to see that it’s anything but smooth.

USM 1 x 500%                             USM 10 x 50%



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