On 11/21/2011 3:16 PM, Chuck Norcutt wrote:
> While I could clearly see the differences in color and brightness I saw
> them only as distinctions without a difference... meaning I really
> didn't care one way or the other. I didn't see A as better than B or C,
> only different.
So you too learned something from the experiment. There are differences in
color and contrast rendering that matter to
others that don't matter to you.
> I assume that vignetting must be responsible for the
> different brightness distributions yet I saw no obvious signs of
> vignetting nor would I have expected there to be such at f/8 and f/11.
No, I don't believe it has anything to do with vignetting. Much as we would
like it to be true that some things be
fairly simply explained by straightforward factors, it's often not the case.
> Rather a mystery actually.
Back when I was a physics student at Berkeley, a couple of centuries ago, I
worked with the equations for diffraction at
edges and refraction across boundaries between substances of different
refractive indices. When we talk about correcting
for the three primary colors, it's easy to forget that it's an analog spectrum
of color going through those lenses.
Even an apochromat lens only brings three wavelengths together at the same
focal plane and only corrects spherical
aberration at two wavelengths. Take a look at this 'S' curve.
<http://en.wikipedia.org/wiki/File:Apochromatic_focal_curve.svg>
Wonderful as lens design has become, it's still a compromise among varying
degrees of many aberrations even beyond those
above. We may think of them as only affecting the image as a whole, or perhaps
only the three primary colors, but
really, it varies continuously across the whole range of visible wavelengths.
To the extent that these aberrations
affect small ranges of wavelengths differently in focus and transmission, they
will affect differently colored areas of
the image differently.
Then there's sample variation. Each piece of glass is slightly different, close
to the same overall refractive index as
those in other lenses and close to evenly refractive throughout, but not quite.
The same holds true for each surface
shape, each coating layer, each inter element distance, centering, and so on.
I suspect those are the reasons that each lens has it's own unique 'signature'
in the images from it.
Moose
--
What if the Hokey Pokey *IS* what it's all about?
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