Reference Guide

page 11
TTL-SIR PHASE DETECTION AF
This long acronym stands for Through The Lens-Secondary Image
Registration Autofocus. It refers to the method employed by EOS
(and most other AF SLRs) to detect the amount of defocus at the
film plane. A simpler way of saying this is that the TTL-SIR
system tells the camera how far out of focus the lens ' is, and in
what direction.
Image-forming light first passes through the camera's main lens
to the semi-transparent reflex mirror. Most of this light is
passed to the camera's viewfinder for the photographer to see,
but the rest of it is diverted to the AF sensor through a sub-
mirror assembly.
At the bottom of the mirror chamber is the opening for the AF
sensing device (for EOS, the BASIS chip). This point also
represents the geometric equivalent of the film plane, so in
reality the BASIS sensor is analyzing a "secondary" image which
is created at this position.
As the light passes through the AF system, it hits a special
prism that divides the image into two components which correspond
in a limited way to the top and bottom halves of a split-image
focusing aid. This type of focusing system is quite common in
manual-focus SLRs. Each component image is projected onto one of
the sensor arrays for electronic analysis.
As long as there is a discernible level of contrast in the
analyzed portion of the image, the AF system can calculate the
degree and direction of defocus almost instantly, resulting in a
digital pUlse-count instruction to the lens focusing motor
through the control circuitry in the lens. EOS exceeds all other
AF SLRs in the maximum amount of defocus that can be detected.
Upon completion of lens travel, the AF detection procedure is
performed again immediately. If the resulting signal indicates
that the subject is in focus, the viewfinder confirmation signal
lights up, and an optional beeper is activated.
If the AF system is unable to find a discernible level of
contrast after one complete search, lens movement is stopped and
the AF symbol in the viewfinder blinks rapidly as an indicator.
ULTRASONIC MOTOR (USM)
Canon's ultrasonic motor is a ring-shaped direct-drive device
which consists of a stator (a stationary element) bonded to a
ceramic element and a rotor (moving element) attached to the
focusing component of the lens and in contact with the stator.
When voltage with a frequency of 20 kHz or higher is applied to
the ceramic element, the vibrations generated by the ultrasonic
sound produce waves which continuously advance around the stator.
When the rotor is placed in contact with the stator under
pressure from a washer spring, these waves apply propulsion to the
rotor and make it turn.