Re: [APML] Focusing OM-1 - but what about the Nikon F2?

[Duncan <dmunro@sfu.ca>]

KGKIRKLEY@aol.com wrote:

>
> In a message dated 11/17/04 9:07:04 PM, dmunro@sfu.ca writes:
>
>
>> Wei-Hao, thanks for the calculations.
>>
>>
>> My NGF-SE focuser (for example) moves the focal plane about 8mm  (8000
>> microns)with a 360 turn of the focus knob. So one degree of  rotation =
>> 22 microns. According to your calculations I need to be able to control
>> the focus knob to about 3.5 degrees of accuracy (3.5deg=77microns).  If
>> I take a 150mm peanut butter jar lid and glue it to knob I now have a
>> focuser with a circumference of about 500mm. If I paste a printed scale
>> with 500 subdivisions on the the lid,  I now have a scale accurate to
>> .72 degrees or about 16 microns of focal plane movement. I have no doubt
>> that I can move the focuser in .5 mm increments so I can effortlessly
>> control the focal plane position to about 8 microns, or 10 times the
>> required accuracy.  I'm guessing that most R+P focusers will have
>> similar amount of focus travel per degree of focus knob rotation, but
>> even if they move the focus position by 100 microns per degree it should
>> still be quite easy to control the focus to the accuracy required.
>>
>> Logically, even with a knife edge focuser you must still be able to move
>> the focuser in small enough increments to  achieve consistent focus. If
>> can achieve consistent focus, then it follows that you can make a simple
>> dial indicator to allow you to repeat the process to the required degree
>> of accuracy, ergo, the knife edge focuser in not needed. All that is
>> needed is a simple indicator to allow you to control the focus position
>> to the required accuracy.
>>
>>
>> cheers
>>
>> Duncan
>
>
>
> Sometimes you can lead a horse to water and still can't get them up on 
> skiis!!!
>
> Kent Kirkley
>
>
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Exactly.

Duncan
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