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Re: [OM] 100 shootout

Subject: Re: [OM] 100 shootout
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Sat, 1 Oct 2011 15:26:14 -0500
The focus helicoid on the 100f2 has around 60 degrees from 1.5M and
3M. So it is just a touch over 15 degrees from 1.5M and the 2M marks.
Plenty of helicoid to work with.

On 10/1/11, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
> 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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>


-- 
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
-- 
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