I've formulated a theory (more of a guess really) about this based on some observations I've made and here it is in all it's wrongness...
A tripod mounted camera will move in a much different manner than a hand held camera. Specifically, it will oscillate rhythmically rather than the rather haphazard movements caused by the inability of a person to hold their hands mechanically still. This of course varies from situation to situation, but given a heavy lens on a underclassed tripod, you can see camera/lens combo oscillate if you tap it (yes I've ordered a better tripod - it's being shipped).
Obviously, the movement of the sensor exerts an equal and opposite reaction in the camera body, and if the tripod allows this force to create movement to some extent, it will of course blur the photo, but it may also create a feedback loop with the sensor trying to compensate for the sensor-induced shake. I've seen (heard) this happen with the camera+Bigma on a weak tripod and it sounded like my camera had turned into a reciprocating saw ("sawzall") - rapid strong vibrations (I thought my camera had died - then I remembered to turn off the SR...) Using proper long lens technique (eye pressed to eyecup, hand on lens above the tripod collar, etc.), dampens vibrations like this and allows for sharp shots even if you forget to turn of SR like I did yesterday (doh!)... Check it out (and it's sharp when pixel peeped too)...
So, there you have it - my theory - Undampened oscillations are increased by a feedback loop between the camera/lens combo and the sensor's movement... Given a smaller setup, say just the camera and a small lens on a beefy tripod but not held or dampened by hand, this sort of thing might not create the full-on feedback loop which caused the mad-wasp buzzing of the sensor, but could still cause more shake than just mirror slap would have... I dunno know if I'm right, but that's all I've got
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