Stephen Johnson Tutorial: Digital Photographic "Sharpening"
(excerpt from the book Stephen Johnson on Digital Photography)
Unsharp Mask Features
Keep in mind that you don’t have to sharpen all three channels of a color file in the same way. Many digital cameras suffer from noise in the blue channel of the color file, which can be exaggerated in sharpening. In Photoshop you can simply select one channel, sharpen it to your taste, and inspect the results. You can also sharpen only selected areas of the image if you choose.
There is no magic set of numbers for “ideal” sharpening. Factors that influence the results are obviously the sharpness of the capture or scan: the resolution of the file, film grain, or digital noise; the paper you are printing on, and naturally, your taste. I generally start with a sub-pixel radius of .3 pixels, then increase the amount until it looks correct to me.
On my high resolution scanning back photographs, I leave the threshold to zero, on film it may go up to 4 or 5 to avoid sharpening film grain. There is no gospel here; you need to experiment and discover the results that please you. Remember, over-sharpening can create image halos around hard tonal edges, outlining these areas in black or white lines.
I use a viewing set-up for the whole image at 100% on screen, and the sharpened preview window at 200%. Additionally, after a sharpen you can Fade the filter (under the Edit menu) through the Luminosity mode and reduce possible emphasis of color artifacts caused by the original sharpen and Bayer-pattern sensors.
I am particularly careful about artifact creation in the "sharpening process". Since most sharpeners are essentially ways of creating light and dark outlines around contrast changes, the key is to create those outlines just below being obviously visible. If a dark outline or white outline has become visible, back off on the amount or radius.
Also be careful of an often internet recommended technique that calls for heavy sharpening with Threshold dialed up to confine the effect to just the edges of the image. This often results in one of the ugliest digital effects I've seen, what I call the oatmeal/razor blade effect, over sharpened edges with what then looks like a mushy blur everywhere else in the image.
Additional Workflow Considerations
Many people duplicate their Background image before sharpening to give them a copy to work on instead of the underlying image. That works fine, but it may be even better to flatten the whole set of editing layers you may have created into a new concatenated layer (command, option, shift E) and work on that layer as contrast and saturation layers can have a dramatic impact on the sharpening.
It is also true that if you bring the RAW photograph into Photoshop as a Smart Object, the Unsharp Mask (and all filters for that matter) become a Smart Filter which can be modified or masked like any other layer.
Classic over sharpening, white and dark outlines have been created
Alternative Sharpening within Photoshop
There are various alternative to Photoshop’s sharpen functions. Some are “hand done,” other solutions can be purchased as add-in filters or Photoshop Actions.
In addition to Unsharp Mask, Smart Sharpen is also a standard filter within Photoshop. I don't generally use it as I find its approach to controls very non-intuitive and based on software engineering metaphors rather than photographic concepts.
One manual technique involves using edge selections as sharpen masks by using the Find Edges filter under Filter/Stylize on a duplicate Layer, creating a Luminance mask, inversing the selection, and sharpening your underlying file through the selection.
Another variation on edge detection and exaggeration is the High Pass Overlay technique.
Be moderate, focus your camera carefully to begin with, use a fast enough shutter speed to prevent camera movement and crank the ISO if needed. Digital "sharpening" is about creating optical illusion, not about sharply focused photographs. The craft remains up front.
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