Sharpening - How it Works

[MBChamberlain]

Let's discuss the sharpening process for a second. This isn't so much a tutorial, but rather a discussion of how all sharpening methods work. It really helps to understand what is going on. Basically I don't advise doing it this way, but going through it manually can really help you understand exactly how unsharp mask works (and how it is somewhat limited).

Try this (in RGB mode)

Make two duplicates of your layer
Reduce the top layer to 50% opacity and invert it
You now have a 50% grey layer.
Run Gaussian blur on the top layer.
Merge down.

You have just done a manual High Pass.

You can run gaussian blur on this if you like to reduce the noise. (threshhold)
Blend this layer in with Linear Light and flatten the image.
Duplicate the layer up to 5 times and reduce the opacity of the highest layer. (If your layers 3 layers with the top layer at 37% your amount is 237%)

Flatten this and you have manually performed an unsharp mask.

Blending with linear light is the killer. It is too harsh. I'll use high pass sharpening, copy it twice and use hard light on one and soft light on the other. (200% sharpening, threshold 0 radius what ever I did on high pass.)

You can run Median instead of gaussian blur to adjust the threshold, you can mix blend modes, adjust the opacity for these blend modes, desaturate the high pass layer to prevent the color halo, to quote every villain from every bad move ever, "I HAVE ULTIMATE POWER."

Probably more information than you ever wanted to know.

Michael

[Flora]

Hi Michael,

thank you so much for your Tutorial!!

I'm surely going to try what you described here as I've also been trying to find a more controlled way to do my sharpening ....

In my opinion, sharpening is a very, very important part of restoring/retouching images and I'd go as far as telling that I think that after the rest of the work has been done, sharpening can actually be the 'make or break' of it .... so .... the more control, definitely the better!!

Thanks again .... I'll let you know ...

[bart_hickman]

Micheal,

Thanks for the discussion!

I thought the threshold in unsharp mask was a non-linear operation--ie., look at the grey layer you make when blending the original and filtered inverse: any pixel that is closer than the threshold to the value 128 gets set to 128. This prevents accentuation of small noise, regardless of frequency. Of course I have photoshop 4.0--maybe they've tricked up unsharp mask in the last 10 years.

What you are talking about sounds like doing a low pass operation (gaussian blur) on the unsharp mask result. The result is effectively a bandboost operation. This prevents the accentuation of high frequency (hopefully out-of-band) noise regardless of amplitude.

Of course these days, the excellent NR software that's available is able to control noise using very selective and/or adaptive filtering techniques.

Bart

[skipc]

This target is helpful to understand the effects of various USM settings. Just zoom in to where the three shades of gray intersect, open USM and try various USM settings...skip