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I know there's hardly any risk to your equipment when dismounting the lens while the camera is on, but can you damage it if you dismount the lens while the aperture is closed down?

With the Canon EOS system it is possible to remove the lens while pressing the DoF preview button causing the aperture to not retract until mounted again. It is useful when using EF lenses on non EF bodies, but can this method harm the equipment in any way?

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Not unless you drop the lens while removing it because you don't have enough hands to hold everything.

Seriously, no you can't damage a Canon EOS lens using this procedure. Canon EOS lenses are designed in such a way that the diaphragm is moved in both directions by a micro servo attached to the diaphragm assembly. Unlike many lens designs, there are no springs associated with the diaphragm in an EOS lens, and the connection between the lens and camera is electrical only. There are no mechanical linkages (unless you count the contact pins for the electrical connections) other than mount itself that holds the lens on the camera. The electrical contacts between the lens and the camera body are also designed in such a way that the main voltage pin on the camera is disconnected as soon as the lens is turned and can not contact any of the other pins on the lens as the lens is rotated to be removed.

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    Very good answer. Does this apply to third party lens manufacturers for the EF mount as well? – Hugo Sep 14 '14 at 19:28
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    Any lens compatible with the EOS mount will operate in the same way. There is no "rest" position in an EOS compatible lens in a mechanical sense. When the diaphragm is stopped down for a picture it only returns to wide open after the shot because the camera sends a command to the servo to open the aperture back to wide open. You can also remove a lens during Live View and the current aperture will be preserved. I don't recommend doing so, however, because both the shutter is open and the sensor is energized during Live view. Though it is unlikely to attract dust from outside the camera... – Michael C Sep 14 '14 at 19:38
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    ...dust already in the light box might be blown toward the sensor by air blowing in the mount opening and would be more likely to stick to a charged sensor. – Michael C Sep 14 '14 at 19:38

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