If all that you say here is true it should be easy enough to demonstrate
with four photos derived from two images shot with different lenses.
One of the lenses should be regarded as having excellent bokeh and the
other with not so excellent bokeh. First show both images at a
magnification such that there is no bokeh since everything in the shot
will be in focus according to the CoC. Then magnify these same images
and view them at such a distance that the original CoC is no longer
sufficient to consider everything in sharp focus.
By your reasoning the result should be that the lens that delivers
excellent bokeh in the enlarged image should make a clearly superior
image in the smaller photo where the CoC still rules and everything
appears in focus.
Show me that and I'll concede that you may have a point. Show me that
for three pairings of different lenses and I'll concede that you are
probably right. Until then I see no reason to accept it.
Chuck Norcutt
On 1/17/2011 1:30 PM, Ken Norton wrote:
>> 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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