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I bought a new Canon 40mm STM lens and in the Lens Manual it says

The focus cannot be adjusted in manual mode.

Does it mean that its harmful to rotate the focus ring when the focusing mode is set to manual mode?

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You need to read the rest of the line. It says

"The focus cannot be adjusted in MF mode while the camera is off."

The STM focus is electronically controlled it would appear, thus you can't adjust it without power being applied to the motors. When you turn the ring, it's just telling the camera to make adjustments with the motors. You don't have an actual link to the focus movement.

The lens also supports full time manual, so if you are shooting in "One Shot" mode, then you can manually adjust the focus after focus is achieved automatically.

  • So do STM lenses use a "Fly by wire" system for focussing?? – Mike Jul 23 '13 at 16:02
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    @Mike - not sure if they all do, but that one appears to since it says there is a lag with quick movements of the ring. That and the power off thing is a dead give away that it is an indirect control via the AF drive. The feature is most common on some of the big telephotos that would be prohibitively heavy to move by hand, but the STMs may be too small to use a mechanical linkage effectively. – AJ Henderson Jul 23 '13 at 16:28
  • At least the 40mm STM is focus by wire. The focus ring does NOT change the focus, it tells the computer that you want it to focus this way or that way. Then the computer tells the motor. Its a great lens for the price, but its not perfect. – Pat Farrell Jul 23 '13 at 17:57
  • All STM lenses are focus-by-wire, as are a few older Canon lens designs, most notably the discontinued and no longer supported EF 200mm f/1.8 L. – Michael C Nov 27 '15 at 2:03

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