Chuck, if I use a tape measure and can concistantly hit the same markings at
three different distances three times in a row, will you be convinced?
On Saturday, October 1, 2011, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
wrote:
> While what you say is certainly correct (the placement of markings along
> the focusing ring is not linear), nevertheless the motion of the lens
> tube itself is linear since the pitch of the helical threads does not
> change. All of my analysis was based on the linear back and forth
> movement of the lens tube, not the non-linear shift in focus distance.
>
> I used the lens maker's equation to determine the linear shift required
> for the lens tube to shift the focal plane over the range of the DOF.
> Since the helical's thread pitch is constant that also has a linear
> relationship with the angular change and linear change measured at the
> circumference of the focusing ring.
>
> I stand by my original analysis. Focus on your same target again at 2
> meters. Mark the focus position very precisely. Move the target about
> 25mm closer or farther and refocus. I'm sure you will find the total
> linear movement of the focusing ring to be less than 1mm. Actually,
> I'll allow you to move the target 38mm or 1-1/2 inches. I still think
> the total refocusing movement will be less than 1mm on the focusing ring.
>
> If I'm wrong I'll buy your dinner when we get to Grinnell. :-)
>
> Chuck Norcutt
>
>
> On 10/1/2011 4:26 PM, Ken Norton wrote:
>> 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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>>>
>>
>>
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>
--
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
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