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[OM] Re: Slouching Toward Something Else

Subject: [OM] Re: Slouching Toward Something Else
From: AG Schnozz <agschnozz@xxxxxxxxx>
Date: Thu, 9 Nov 2006 14:57:34 -0800 (PST)
> I'm afraid you'll have to convince the skeptic (me) on this
> one.  How can it know the distance to various autofocus
> points without having focused on each one and gotten feedback
> from the lens positioner?

Each manufacturer has their own patented smoke-and-mirrors to
accomplish this, but here goes:

Assume just two active focus points for simplicity sake.  Point
One is in focus and the distance is close to infinity. The
camera actually knows the relative position of the focus
distance (That racking in and out when you turn on or mount the
lens actually identifies to the camera the full focus range) and
in some cases actually knows the specific focus distance.  Point
Two is out-of-focus. Unless the lens is a long-telephoto with
focus-limit controls, the camera immediately makes the
assumption that Point Two is closer than Point One and will
readjust focus.

Ok, now the camera knows the relative distance (usually as a
percentage of the entire focus range) between Focus Point One
and Focus Point Two.  The matrix-metering now assumes that Point
One is background and Point Two is subject. By taking a meter
reading of these two points the camera has effectively built a
3D matrix of foreground/background.

Depending on the camera (Minoltas and the F5 being examples),
the camera may adjust the focus back-to-front when in "Single"
mode. This builds a 3D profile of the entire image and knows the
relative distance position of each sensor in relation to the one
focused on the closest point.  By applying the exposure matrix
to this, you can see where a tremendous amount of logic can be
applied to the data and if color information is known (like in
the F5) additional judgements can be applied.

My understanding of how this works when you have the focus set
in "Continous" is that during the initial shutter-release press
that the camera will typically initialize the focus at infinity
and then lock on to the closest AF sensor point.  The camera
will then track this across adjacent AF sensor points.

Sensors aren't just dumb rocks either.  For example, in the
Canon, if the subject is closing in on the camera location, and
the AF mode is set to continous, it doesn't "hunt" the wrong
way, but actually will track the subject as it comes closer.
This requires the sensor to have the ability to differentiate
between closing and departing subjects.  My E-1 is a dumb rock
in this, though. :(

Anyway, that is kinda an overview of how 3D matrix metering
works.  Each manufacturer has their own specific way of making
it happen and they all take a lot of shortcuts and assumptions.
For example, as long as the camera knows the relative focus
position of the lens it will assume all out-of-focus AF points
are background.

AG


 
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