Revision: 20130616_013217 | All Revisions
Canon EF 40mm f/2.8 STM Lens
Instead of going with the established and highly acclaimed UltraSonic Motor (USM), Canon decided to use its new STM technology in this lens. The reason is because
Whereas the USM is a combination of mechnical and electrical mechanism, the STM is practically an electrical system. With USM, the manual focus ring is always mechanically engaged to the focus mechanism. That means you and manually adjust focus after AF is achieved without changing the mode switch.
On the other hand, with STM, you are either in AF or MF mode. Rotating the focus ring in AF mode does absolutely northing. Switching to MF mode does not engage any mechanical connection between the focus ring and the focusing motor. In MF mode, the focus ring controls the motor "by-wire", similar to the concept of "drive-by-wire" in modern cars. This means when you rotate the focus ring, the camera computer senses your adjustment, then adjusts the focus motor to match. But even with this mechnical disconnect, manual focus feels precise and responsive.
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