> Agreed but that has nothing to do with bokeh.
LOL, well, I guess I'm feeling my oats today....
I believe that how a lens renders the visible bokeh determines how the
in-focus items are rendered. I have been pretty well convinced that modern
lenses with aspherical elements as well as focal-length corrective optics
(zoom lenses) fold the out-of-focus images inward which can form donuts or
hard-edged highlights without any form of penumbra.
But, you ask, how does this relate to in-focus? At the exact plane of focus
it would have zero effect. Other than aberrations, every lens should render
the plane of focus quite similarily. However, it is the in focus zone either
side of the plane of focus, which is within the mathematically calculated
margins of the DoF where the differences occur.
With most modern-era lens designs, we have exceptionally sharp images within
the DoF because the rate of out-of-focus blooming is greatly restricted.
This restriction is caused by the folding in of the penumbra back on itself.
Some lenses, however, have a distinct bilateral expansion which yields a
quadrupal image overlay of anything out of focus.
So, an old, classic lens-design is generally one which has a linear
expansion of the bokeh, by spreading the image over a larger area. As this
spread happens, the contrast range of the out-of-focus areas of the scene
will decrease as subjects fade into each other, blending and averaging out.
But through subsequent generations of a lens design, this softening effect
(penumbra) is decreased because it does affect the apparant sharpness of the
image. By keeping the penumbra small, you increase the contrast and
therefore edge detail when near the plane-of-focus. This translates to an
overall sharper lens at the expense of the penumbra. Ultimate resolution is
not increased, but the contrast is. This translates into an apparantly
sharper image.
A little explanation of term:
Penumbra - The transition zone at the edge where the object blends into the
background.
An illustration of what I'm talking about is the Trillium (flower) picture
on this page:
http://www.image66media.com/page24.html
Notice how the edge of the out-of-focus leaves do a soft blend into the
background. Also note how the background leaves are of a lesser brightness
value than the Sepals which are in exactly the same light and are
essentially the same color and tone as the leaves. This is caused by the
averaging of the values in the background as well as a general light-falloff
due to magnification.
Related to all this is another issue which many of us encounter without
understanding the root cause. The older lens designs with linear penumbra
expansion do not produce images that subject themselves well to USM
sharpening. USM and variable penumbras don't mix well and you'll get
stairsteps in the tones or halos on some edges, but not on others. With
modern lens designs that restrict the penumbra and force the bokeh blobs
into hard-edged affairs, most edges within the image have similar softening
regardless of how far away from the plane-of-focus subjects may be. When
applying USM to a modern-lens image, the edges respond equally to the effect
and halo suppression is easier.
But that's not why you called...
AG
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