Yup. Does that mean I owe you dinner? :-)
Chuck Norcutt
On 10/1/2011 8:39 PM, Ken Norton wrote:
> 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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>>
>
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