He may have had it figured out to begin with. I just have a great deal
of difficulty coming to grips with accurately controlling the very tiny
motions of the focusing helicoid. If he tells me he's been using a
focusing rail all along then I'm a believer already. The linear
mechanical motion via the rail is about 25-40 X greater than the linear
motion along the circumference of the focusing ring to change focus by
the same amount.
Chuck Norcutt
On 10/1/2011 2:49 PM, usher99@xxxxxxx wrote:
> Dr. Focus fleshes out some of the experimental design difficulties.
> It is likely much easier to slightly move the cam on a geared head or
> geared
> focusing rail then play with the the focusing helicoid. Suspect the
> tiny change in lens to subject distance won't
> matter much if the test is designed properly. DXO does this to ensure
> optimized focus (they had difficulty with that a few years back)
>
> http://www.dxomark.com/index.php/About/In-depth-measurements/DxOMark-testing-protocols/MTF
>
> The previous version of this page was more descriptive. They take many
> images and calculate their "Blur Index" at several apertures and use
> the best
> as the one with the optimized focus. (some Zeiss lenses can get
> hammered at some focusing distances due to purposely left field
> curvature--doesn't make them an "inferior" for many purposes however)
>
> Cognisys Stack shot automated focusing rail for focus stacking claims a
> precision down to 0.01mm if desired when moving the cam.
>
> Iterations are your friend to average out the focusing differences,
> Bottom line, I still think AG's conjecture that there may be a
> discernable dof or distribution of dof difference between the two
> lenses is a testable hypothesis w/o making it into a Ph.D. thesis.
> There is commercial software where one inputs the lens elements,
> spacing etc and it will provide ray traces and computation of
> aberrations.
> I am not aware that it does full wave dof calculation though Zeiss
> clearly has the software.
>
> http://skyscientific.com/
>
> AG is a superb engineer, he'll figure it out.
>
> Mike
>
>
>
>
>
>
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