Archived posting to the Leica Users Group, 2002/04/27

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Subject: [Leica] How does AF really work?
From: Javier Perez <summarex@yahoo.com>
Date: Sat, 27 Apr 2002 23:32:16 -0700 (PDT)

Hi Everyone
As you may all know, for the past few weeks I've
playing around with EOSes in an unsupervised
environment. I've got the concept more or less nailed
down. The sensor responds to changes in contrast and
the lens focuses in or out until that max value has
been reached or perhaps trapped predictively and
attained. That's simple enough but assuming a static
situation at the time when the sensor is activated,
how does the camera know which way to  turn the lens?
At first I assumed that it was turning a tiny
increment and measuring contrast change from the
initial position and deciding from that whether to
keep turrning or stop and turn the other way. But
there is no backtracking whatever on the barrel and it
always turns in the right direction. Somehow the
camera knows whether the object is closer or farther
from the initial focus point.  The focusing barrel on
eos lenses does have a VR slider which can be used I
guess to inform the camera on the distance of the
focus point in order to allow the thing to land on the
right spot without bounding but it still can't tell
the camera whether the object is ahead or behind the
focus point. The only other idea I can come up with is
that a separate sensor working like a split image can
figure it out the same way you would  based on the
position of the split images.*But that sounds wacky
and trouble prone. Also, It can't be measuring
differences in contrast between closely spaced objects
and acting on that because it can AF properly on
anbsolutely flat expansive surfaces as long as there
are vertical lines. And as far as I know it's not an
active system with respect to the outside environment
either. The info I've gotten off the net is
superficial and I'm still stumped! Does anybody know
how the camera figures out which way to turn?
Javier



* On most split image screens the top image towards
the right and the lower image towards the left
indicates that the lens should be turned CCW from the
shooter's standpoints and that the object is farther
away. I think it's the standard. Though some companies
did it backwards for reverse helicoid lenses like some
of the old Schneiders used on Exaktas requiring a
reverse split image. Finally, some had a lens or
screen reversed and you have to turn backwards. Double
pain in the ass!

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