Author's note: I'm scheduled to do two two-day workshops at the Mesilla Digital Imaging Workshops at the end of January. One will be on photo restoration, the other on using Layers in Photoshop. This column is a taste of what participants will be getting in the latter class. We still have room for more students; I hope some of you will be inclined to sign up.
I recently stumbled on a very useful and flexible way to enhance detail and sharpen up images. I'm sure I'm not the first to hit on this; still, Martin Evening (a walking encyclopedia of Photoshop skills) gives it only a page in Adobe Photoshop CS3 for Photographers and seems not to realize how very useful it is.
In short form, it works like this. Duplicate your image layer. Apply Photoshop's "High Pass..." filter to the top layer. Set the layer blend to "Soft Light." Detail's enhanced!
Long form: I hit upon this trick while trying to improve the eclipse photo above. The original image is more than a dozen times larger than the one here. The coronal streamers I wanted to bring out are broad features. Normal edge and fine-detail boosts miss them entirely; the local contrast enhancement method I described previously improves detail only slightly.
The High Pass filter lets me set the width of the filter. In this case, the radius was about 60 pixels! Doing that got me the version below (kinda—I painted the disk of the sun black by hand). The effect of the filter is to produce an image where non-detail comes out a uniform 128-grey. Edges get brighter or darker. So what?
Well, "Soft Blend" is one of Photoshop's "weird" blending modes. When you blend a layer that way, any pixels with values of 128 have no effect on the underlying image. Pixels lighter or darker than that lighten or darken the underlying image.
See where we're going? High Pass + Soft Blend exaggerates edges. Now, so do most Photoshop sharpening tools, but the cool part of this method is how much you can control the enhancement, as well as apply it to features far too large for normal sharpening methods to notice.
By itself, the High Pass layer has a puny effect. You can make the effect stronger by blending with one of the other modes, all the way up to "Linear Light." A Curves adjustment layer associated with the High Pass layer is a much better tool. The screen shot above shows the arrangment of layers and the curve I used to get me to the second picture at the top of this post (the finished eclipse picture). What that curve does to the High Pass layer is shown here:
Lock down the 128,128 point; then you can do anything you want to the rest of the curve without altering overall image brightness. Here I just kicked the contrast way up. But, for example, if I'd wanted to enhance the dark side of the edges and not the bright side (to avoid bright haloes, perhaps), I could have drawn a curve that looked "normal" to the right of 128,128.
You can manipulate the results other ways. Want different strength enhancements in different areas? Airbrush black into the High Pass or Curves layer mask channel to reduce its effect. Want no enhancement in some areas? Paint over those parts of the High Pass layer with 128-gray. Want wildly different enhancements in different areas? Make multiple High Pass/Curves layer sets and paint in the masks to restrict the impact to the areas you want.
This approach works fabulously well with digital camera images. Try it for yourself, with a filter radius of about 2 pixels to start. You'll be amazed how much you can sharpen your photos without getting annoying edge artifacts. Better still, if 90% of the photo looks great and 10% shows artifacts or halos, just paint those areas down in the masks. It's merely my personal taste, but I feel this approach is giving me the most natural-looking sharpening I've ever gotten.
Warning: High Pass sharpening also greatly enhances noise and grain. You'll want to run a noise reduction pass on the original image before filtering and on the resulting High Pass layer to keep it under control. I used the Neat Image plug-in in this example; Noise Ninja's also good.