How on earth does High Pass Sharpening make an image sharper??

[Ezookiel]

Just found instructions on doing High Pass sharpening in Photoshop, while I was reading a Canon Tutorial on how to take wildflower photos.
It blows my mind how the hell that makes things sharper. How does laying a weird grey image, over the top of the normal image, make it look so much sharper?

I an NOT asking HOW to do High Pass Sharpening. Nor am I disputing that it doeswork. I know how to do it. In fact it's now probably my favourite way of sharpening an image, but it just seems like some kind of "black magic" that a weird greyed out image layed over the normal image, actually sharpens the main image. Talk about a WTH moment. How is a grey image over a normal image making it sharper. I'm a bit lost here.

For those new to all this like I am, who don't even know what I'm talking about, it's a method of sharpening where you take an image, create a duplicate layer, do a High Pass filter on that layer (getting the pixel radius right seems to be the secret - start at around 10 pixels and work up or down from there) and then make it an "overlay" on the original image at about 30-50% transparency.

For some reason this weirdly grey looking image now laid over the original makes the original look so much sharper. How the hell? What the heck? It's black magic I tell ya. It's the devil's work.

[JM Tran]

Yeah it was pretty cool when I first tried it out, really should use it more though!

[Ezookiel]

I can't help but keep asking "Why is it so?"
I'd love to know the technical aspects of just HOW it makes the image sharper. It makes no real sense to me that a mostly grey image overlaid helps.

[ricktas]

High pass sharpening works by evaulating the edges (differences in contrast along edges) and sharpens those areas, rather than the whole. So it would leave skin fairly much alone, but when it hits the eyes it sharpens, cause it detects the edges of the eye lids. pupil, etc.

Have a look in the AP tutorials forum, there is a thread there about why all digital photos need sharpening, and includes the high pass method and how it is done.

[Dandelion]

With an extra layer of grey, the half-tones you would normally see will disappear and the edges of an object become more defined. It makes a lot more sense when you look at it in full zoom mode with Photoshop.

darn, sniped.

[Ezookiel]

Yeah, I saw that thread. It shows how to do it, but not really how it works. I'm guessing what is actually grey in the overlay must actually be transparent, not actually grey.
It's certainly impressive. I've never been too thrilled with the results of some of the sharpening methods around. But I tend to try several methods on an image till I find the one that works best for that particular image, so having this extra method up the sleeve certainly helps, but it just seems so contrary that the weird looking High Pass image, laid over the normal image, would sharpen it. I keep hearing Julius Somner-Miller in my head saying, "Why is it so?"

[mikec]

Is this similar to using the Masking slider in the sharpening section of Lightroom? From what I've read it seems like it's a similar process, just done manually?