USM lenses are manufactured by Canon.
Canon manufactures USM lenses; the letters USM stand for "Ultra Sonic Motor." These are small motors built inside the lens that drive the different lens groups during the focusing process. Canon designed these motors to be extremely quick and very quiet. USM lenses generally carry the "L" designation that identifies the professional grade of Canon lenses.
Cost
USM lenses cost more than other Canon lenses as of 2011. The increased cost reflects not only the more expensive USM motors, but the improved glass used in the lenses. USM lenses offer improved weather sealing against dust and moisture due to the coatings applied to the glass elements and to the resulting construction of the lens.
Performance
USM lenses are faster focusing lenses that are designed for the professional market. These lenses will not only focus faster, they also have larger minimum apertures, generally f2.8 or lower. By letting more light in, the photographer can use a faster shutter speed. These lenses are therefore referred to as "fast" lens.
Identify
USM lenses are identified two ways. The professional "L" grade lenses have a red ring that wraps around the outside of the lens near the front of the lens; the ring is a decorative touch. The letters USM will appear around the outer ring at the front of the lens.
Manual Focus Override
USM lenses not only focus faster but they allow for quick manual focus adjustments, even when the lens is set to auto-focus. Non-USM lenses can also be focused manually but require the lens to be switched from auto-focus to manual focus.
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