When you apply USM (unsharp mask) to an image that contains gradients, what
can happen is the change from a smooth gradient to a stair-step.
Depending on the lens design, focal-length, and aperture, the bokeh of an
out-of-focus highlight will look like an balloon with a soft edge.
Depending on these three factors, the transition between what I will refer
to the "umbra" (solid balloon) to the "penumbra" (the fade-zone) is what
makes or breaks the bokeh.
Some combinations will yield a wide penumbra, whereas others have nearly no
penumbra. Sometimes, you will encounter a lens-aperture combination where
the penumbra wraps back onto itself and you'll get a bright rim around the
balloon. This is very common with wide-angle lenses with asymmetric lens
elements where they induce astigmatism when shot wide-open. In some extreme
cases, with wide-angle zooms, you'll actually get donuts.
A lens, which is "symmetrical" in design, such as those typically used on
large-format cameras will usually have have the penumbra starting
immediately from the center of the OOF (out of focus) highlight) and will
have a relatively smooth roll-off all the way out.
If you apply a bit too much USM to a photo, a lens which produces a narrow
penumbra will show a relatively smooth, but short transition without USM,
but will "donut" with USM. Backing off USM from "donut mode" will create a
harsh step and essentially eliminate the penumbra.
AG
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