Olympus E-3 User Manual page 67

light to an AF sensor at the bottom of the camera. A set of
prisms send the light rays coming from different sides of the lens
to the AF sensor array. This splitting of the image components
creates, in effect, a triangle with a base identical to the lens'
diameter.
The two sides of the "triangle" are then analysed and the light
patterns (peaks and valleys) and the phase difference (distance
between the graphical features when measured from the side
axis) is calculated in order to find if the object is in focus or not.
By analysing the "length" of the sides of the triangle it is possible
to determine if the lens is focused behind or in front of the
object. Using this data, the camera's computer then determines
the direction and amount of focus adjustment needed to correctly
focus the subject.
The E-3 was, at launch, unique among professional D-SLR
camera in that all of its sensors were cross-type sensors. In this
arrangement, there are two sets of sensors for each focus area
arranged at 90 degrees to one another. This permits the same
precision and speed of focus no matter whether the camera is
held in landscape or portrait mode.
Some later competitor models have now added cross-sensor
array technology to a selection of the sensor points but, at as at
the date of writing, the E-3 is still the only professional D-SLR
that has cross-type sensors at all focus points.
An added boost to the focus speed is provided in some of the
newer top-pro lenses in the Zuiko range in that they have SWD
Supersonic Wave Drive
(
sound pulses to activate the rotational motion of the lens
mechanism rather than a miniaturised conventional electric
motor. (The operational principles of these motors are beyond
the scope of this work but if you must know more look up
"ultrasonic motor" at www.wikipedia.org.)
66
) motors. These use high-frequency