Autofocus (Af) - Olympus OM77AF Instructions Manual

Autofocus (AF)

TTL "Zero-In" System
An exceptionally accurate TTL "Zero-In" elec-
tronic focusing system, which works on the princi-
ple that image discrepancy is zero when an object
is correctly focused, is employed by the OM77AF.
Light passing through the focus-mirror is passed
through an infrared cut filter, a correction lens and
then projected onto a CCD "Zero-In" sensor where
it is converted into electrical signals. Using these
signals to determine the image discrepancy, a CPU,
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by reference to a lens-mounted ROM, calculates
the appropriate lens rotation and transfers this
information to an AF motor. This single calcula-
tion/focus correction sequence results in quick and
accurate focusing. Additionally, when the Power
flash Grip 300 is connected, the focus information
is used to provide precise distance information for
perfect flash-exposure control.