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Monday, 27 October 2008

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The technique's been around for a while, but is definitely terrific. Try Overlay mode too.

Ctein -

Great tip. I have been using this technique for some time and it is really useful. Switching to different blend modes can give differing results as well, as you said. You can also manage noise to an extent by masking - so that only parts of the photo that really need sharpening are effected.

Ed

The only place I found such a discussion is in Bruce Fraser's «Image Sharpening» book (pages 135-138). Although people like to have one button, or one technique, for most processing, this shows that the preferred masking approach (in the book and in Lightroom) is not the best for all kind of photos... methinks here is one where the mask would induce a weird texture on the low dynamic range outside of the inner circle.

Very nice to see a good example put to practice as my photos are rarely with such properties.

As for noise reduction, perhaps a mask for blurring the non-detail areas could work to reduce that effect.

How does this compare to running USM with a 60px radius?

Ctein,

I have a friend whose job it is to edit astronomical images for public outreach; he uses the high-pass filter exclusively as a sharpening method (with various blending modes, of course). Don't talk to him about USM, he ain't buying it :-)

One of these days I'll have to ask him to explain it to me slooowly. Or wait for you to give a seminar in the Boston area ;-)


--M.

I've been using this exclusively for about 5 years as a way to sharpen my 4000 ppi scans of 120 film. I set the dupe layer to overlay as Chris does. You can adjust the radius and opacity to suit the particular image. Nice thing is, your images look sharp, but never sharpened, as in that artificially over-sharpened look that seems ubiquitous on flickr and other sites. It works wonders on digital capture files also.

I see fernando beat me to it. Real World Image Sharpening with Adobe Photoshop CS2 does indeed have three pages dedicated to high pass sharpening. Those pages concentrate on using it with the Overlay mode. At 60px I think you're getting into midtone contrast rather than sharpening (to pick a few nits) although that sort of depends on what resolution image we're talking about as well. Sharpening can be such big world. But then again, is it true that "Sharpness is such a bourgeois concept"? I wonder if HCB really said that.

I have used this for a while with portraits and it is great for selective sharpening. You use the high pass filter and then with a brush set to 50% gray paint out the areas you don't want to be sharpened like flaws in the skin. Then the areas around the eyes and nose and other facial features will be sharp. Then set it to soft light.

It's also worth noting that, for most images, it helps to reduce color noise artifacts by desaturating or converting the highpass layer into black and white.

Dear Folks,

Which blending mode you use is a matter of personal taste. Overlay works, as does Hard Light, et cetera. They're all stronger than Soft Light. My working style is to start with a minimal effect and then increase it until I get the degree of sharpening I want. So starting with Soft Light and using a curves adjustment layer to increase the contrast of the high-pass layer suits my habits well. But one could just as easily start with a very strong blending effect and use a curves adjustment layer to reduce it to the desired level. Six of one, half a dozen of another.

High-pass filtering does not work the same as unsharp masking, Especially not with large radius filters. Large-radius unsharp masking magnifies the difference between every pixel and the average of the surrounding pixels. This is good for enhancing local contrast; it doesn't particularly select for edges. Used at a strong enough level to produce noticeable edge enhancement, it can badly distort other tones. In the case of the sample image above, a low level of unsharp masking is good for bringing out the tonality in the corona, but it does almost nothing to bring out the edges of the coronal plumes and streamers. High-pass filtering and unsharp masking are two different tools with two different purposes.


~ pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]
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To avoid enhancing noise when using high-pass sharpening, just run a Gaussian blur on the HP layer with a small radius (typically between 0.5 and 2 pixels) and "voilà": all details of the order of your high pass sharpening radius will be enhanced while the noise will be blurred out.

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