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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