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. However, at least
with Canon, the lens does have a closed loop where it verifies that
the instructions from the body was accurately carried out. If you
have any references that indicate that Olympus does use a feedback
system I'd be interested in reading them.
As to the Canon system, the following information is quoted from a
White Paper written by Chuck Westfall of Canon USA:
"When you mount a lens (whether the camera is on or off), the camera
interrogates the lens for its characteristics, including maximum
aperture, which one of the focusing parameters.
When you half-press the shutter release (or the * button, if you've
used the custom function to move focusing control there), the
activated AF sensor "looks" at the image projected by the lens from
two different directions (each line of pixels in the array looks from
the opposite direction of the other) and identifies the phase
difference of the light from each direction. In one "look," it
calculates the distance and direction the lens must be moved to
cancel the phase differences. It then commands the lens to move the
appropriate distance and direction and stops. It does not "hunt" for
a best focus, nor does it take a second look after the lens has moved
(it is an "open loop" system).
If the starting point is so far out of focus that the sensor can't
identify a phase difference, the camera racks the lens once forward
and once backward to find a detectable difference. If it can't find a
detectable difference during that motion, it stops.
Although the camera does not take a "second look" to see if the
intended focus has been achieved, the lens does take a "second look"
to ensure it has moved the direction and distance commanded by the
camera (it is a "closed loop" system). This second look corrects for
any slippage or backlash in the lens mechanism, and can often be
detected as a small "correction" movement at the end of the longer
initial movements."
Later,
Johnny
__________________________
Johnny Johnson
Cleveland, GA
mailto:jjohnso4@xxxxxxxxxx
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