Olympus E-3 User Manual page 66

Still with us so far? Good. Grab another cuppa and a re-fill of
nibbles and let's investigate the many focus options crammed
into the E-3 camera. As always, however, a little bit of
background can do no harm...
AUTO FOCUS 101...
Auto focus lenses are, perhaps, the single greatest mechanical
refinement applied to the art of photography since Mr. Bernac
invented the portable 35mm camera.
It is inconceivable to think how any of the iconic sporting and
news images of recent years could have been produced without
the help of auto focus technology.
O.K. Point taken but how does it work then? It is all really rather
simple if you have the computational power available - which any
modern camera has in spades...
The auto focus system of modern D-SLR cameras is usually
based on what is known as the "Phase Detection" system. Some
systems use a more easily understood description called
"Contrast Detection" but, in essence, they both make use of a
graphical representation of the light and dark patterns in the
image.
Phase detection, however has the advantage that it "knows" how
far out of focus a subject is and which way to drive the lens to
achieve focus in the shortest possible time. Contrast detection
systems, however, need to hunt around a bit to find out where
the point of focus is located.
Phase detection operates by dividing the incoming light into
pairs of images and comparing them. A beam splitter (normally
housed under the main mirror which is semi-transparent) directs
FOCUS
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