Topic: PS/CM method for "finding-sharp-areas"?

[KlausNordby]

I have a need for some PS/CM method for "finding-sharp-areas" -- like the Masking preview you get with Alt-drag-slider in Lightroom 2.  I've fiddled around with various filters and Difference blend mode, but haven't yet come up with any really useful method.  Would any of you clever folks here have any good suggestions?

K

[curvemeister]

One way to find sharp edges is to subtract the image from a blurred version of itself.

1) dup the image to a new layer
2) blur it
3) subtract it from the underlying layer
4) blur the result, and use levels to bump the contrast.

This can be used, for example, as a mask to combine two images, each focused at a different distance.

[KlausNordby]

Thanks, Mike. But "subtract"? I knew of no such blend mode or other command, till I tried the PS help -- which told me that Add and Subtract blend modes were hidden in the Apply Image and Calculations dialogs ONLY. This was news to me, I must confess -- after 18 years of PS use -- two new blend modes never seen before! And yes, your recipe does seem to be a viable route to what I need, so thanks.

[curvemeister]

I meant to say "difference" instead of "subtract", though either one should work.

Glad the technique seems suitable. 

[KlausNordby]

Hmmm.  Mike, so far it seems that your "Apply Image: Subtract" blend method doesn't give me nearly as good sharpness masks as the Masking preview in Lighroom.  They are just brilliant -- and very flexible and easy to do with the Alt-drag-slider.  Alas, I see no easy way to get that mask out of LR and into any other app  (or maybe I'm happily wrong here?).  So I simply vote that you write us such a nifty sharpness-mask-creation method inside CM.  ASAP. :-)

(Addendum to your addendum: I've previously tried blurs and the Difference mode, but that didn't do it for me.)

[KlausNordby]

On further tests with the Masking preview in LR, I see I am somewhat mistaken: it doesn't separate out what's really in sharp focus from what's more blurry.  If there are blurry edges but with high contrast, the LR Masking includes them.  So it's not what I need for a true "finding-sharp-areas" filter, and Mike's Subtract method has similar problems.