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Re: [OM] 420 deal

Subject: Re: [OM] 420 deal
From: WayneS <om3ti@xxxxxxxx>
Date: Sun, 21 Dec 2008 11:23:11 -0500
Then I stand corrected. I made an assumption that the
system was closed loop.

Any open loop system can produce all kinds of errors if the
predictive algorithm for a specific lens is not correct. I would
also suspect that the problem would show up more in wide
angle lenses, where small focus increments create larger
focus errors. Also lens focus hysteresis would be a problem.
The lens could front focus or back focus depend on direction
it needs to move.

Wayne

At 11:04 AM 12/21/2008, Johnny wrote:
>At 08:40 AM 12/21/2008, WayneS wrote:
>
>>As C.H. alludes to, this should not be a cause as the auto-focus is
>>a servo feedback system. Only if you take the shot by fully pressing
>>the shutter release, such that the system fires as soon as it get AF
>>indication, should focus over or under shoot be a problem.
>
>Hi Wayne,
>
>The auto-focus on Canon DSLRs, and I suspect also for Olympus, does 
>not use what I would call a feedback system.  The phase detection 
>system takes one look, makes a calculation as to front or back focus 
>and amount of focus error and then instructs the lens as to direction 
>and amount to move to correct the error.  The focus isn't re-checked 
>after the lens reacts to the initial instruction.  <snip>



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