Reference Guide

page 2
ARC FORM DRIVE (AFD)
The Arc Form Drive is a small but powerful electromagnetic motor
used in most EF lenses as an autofocus actuator. The drive unit
itself is arc-shaped to match the standard cylindrical
configuration of a camera lens. The AFD is a modified Hall-type
brushless stepping motor, which means that it possesses ample
torque and extremely good start-stop response compared to rotary
focusing motors found in body-integral AF SLRs. Also, AFD motors
have superior resistance to wear and can be manufactured in a
variety of sizes to custom-fit the needs of many different lens
types.
The AFD drive shaft rotates at speeds from 6,000 to 12,000 rpm
and transfers its power through a set of reduction gears to
either a helicoid or cam attached to the focusing component of
the lens. During AF operation, the AFD is disconnected from the
lens's manual focusing ring, thus preventing unnecessary
resistance even if the photographer's hand is touching the ring.
Changing from AF to manual focusing is accomplished with a simple
switch on the lens.
ASPHERICAL LENSES
Aspherical (non-spherical) surfaces play an important role in
Canon lenses. They provide maximum sharpness at all apertures
because they can focus all incoming light at the focal plane (a
theoretical impossibility with spherical lenses).
Another important benefit of aspherical lens elements is that
they minimize flare (non-image forming l~ght caused by internal
reflections), particularly with high speed lenses.
A third benefit is that often, the number and size of lens
elements in a given optical formula can be reduced. As a result,
the size and weight of aspherical lenses is usually less than the
equivalent spherical counterpart.
Aspherical formulas are particularly useful with short to medium
focal lengths and/or wide aperture lenses. There are 10
aspherical lenses in the EF line, ranging from the 28mm f/2.8 to
the 70-210mm f/4.
Canon uses two manufacturing techniques for aspherical lenses.
The ground and polished method is used primarily with high speed
or pro£essional L-Series lenses including the 50mm f/1.0L and the
85mm f/1.2L.
The other method is called glass molding. In this procedure, an
unfinished element is placed in a special aspherical mold. It is
then rapidly heated, pressed into the mold, cooled, and polished.
Both methods produce top quality lenses. However, glass molding
technology is currently limited to lens elements with diameters
of 40mm or less and is most suitable for high volume production.
The ground and polished method can accommodate larger diameters,
and therefore is more suitable for high speed professional
lenses.