Canon EOS 50D Instruction Manual page 291

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When you want to photograph a small animal or
an insect, it's important to get close quietly -
without scaring the subject. Canon Macro Lenses
with USM and inner focus let you photograph
subjects with quiet, rapid autofocus. You'll feel
confident to approach many subjects that are
sensitive to sound and motion in the big world
around them. You'll be able to get close and
capture big, bold images of small worlds!
This stands Ultrasonic Motor. Canon has pioneered development
of USM for camero lenses. In we've optimized autofocus
action for responsive
\Nith
high vvh!le
minimi,:inq motor noise.
Technology octivotes focusing by shifting a group
of
lenses inside the unit, rather moving (expanding or contracting) the
whole lens unit Since the overal! length of the lens doesn't change. you can enloy shooting ease and compact dimensions.
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