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Sigma's new 20-40mm f/2.8 EX Aspherical DF lens combines a constant f/2.8 maximum aperture with ultrawide-to-wide focal lengths in a new design which is easier to handhold. The lens incorporates a "Dual Focus" mechanism, which makes the focus ring stay stationary during autofocusing. Unlike most other autofocus lenses, torque of the focusing ring during manual focus remains excellent. A helical focus system eliminates front lens rotation.