How does chdk (internally) do focus bracketing? - General Help and Assistance on using CHDK stable releases - CHDK Forum
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How does chdk (internally) do focus bracketing?

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Offline raocemail

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How does chdk (internally) do focus bracketing?
« on: 20 / May / 2011, 09:37:50 »
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How does chdk (internally) do focus bracketing?  That is, I can see how the focus can be changed by 1mm at a distance of 2cm, but cannot see how it is possible to change focus by 1mm at 200m.  A corollary is, how small a change is practicably possible at a given distance?  (Camera model is SX120is, in case you need it.)  Thanks.
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Offline Microfunguy

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Re: How does chdk (internally) do focus bracketing?
« Reply #1 on: 20 / May / 2011, 10:55:39 »
Hello and welcome.

What actually changes is the lens stepping motor position.
It changes focused distance by a tiny amount when focused very near and by a very large amount for distances more than one metre, especially at the wide-angle setting.

SDM has a built-in feature that will log the focused distances to a file as it automatically captures images from the initial focus to infinity.

See examples in 'Automatic capture of images from focused distance to Infinity' here http://stereo.jpn.org/eng/sdm/capture.htm
« Last Edit: 30 / May / 2011, 18:47:15 by Microfunguy »
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Offline raocemail

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Thanks, I understand, but still have a question.
« Reply #2 on: 29 / May / 2011, 10:45:45 »
Appreciate the reply.  However, I was not able to access the link in the reply; apparently it is invalid.  I do understand the concept that what changes is the position of the focusing stepper motor.  To me it makes sense that a given change in position of the stepper motor corresponds to a small change in focus position for objects nearby and to a large change for objects in the distance.  That would be SIMILAR to saying a certain amount of change in position of the stepper motor corresponds to a certain percentage change in the focus distance.  The reason I have a question is that as far as I know with chdk one specifies a change in focus distance in millimeter.  That is an absolute change, not a percentage change.  It is that discrepancy that I do not understand and that prompted my question.  Again I thank you and would appreciate your kind reply.
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Offline Microfunguy

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Re: How does chdk (internally) do focus bracketing?
« Reply #3 on: 30 / May / 2011, 18:49:49 »
The above link is now corrected, please try again.

I do not see the point of the CHDK fixed-increment focus change.

Whoever implemented it that way does not understand photography.

It would only apply to a limited range at a very close distance.
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