This is something worth looking into.
I've had my 100-400 on a tripod, attached by the collar, and seen quite obvious wobbling as a result of the mirror slap and shutter release. I recall on a number of occasions looking through the viewfinder as I pressed the shutter with remote cord and noticing pronounced image shimmer immediately afterwards.
This tripod is very light, weighing just 2.2Lbs or about 1Kg and very compact. It is recommended for a maximum load of 5.5lbs and is described as being suitable for digicams. If mirror slap is going to have an effect, then this is the tripod that will reveal it, (or do you think my logic is flawed?)
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Interesting results Ray; I don't see any difference between the images. The 50mm lens is too good though! If this test would be standardized in any way, it would have to specify a 200 or 400mm lens and a reasonably long target distance that can still be set up indoors (for controlled lighting and "clear weather"). I saw your followup post with the 100-400 lens at 1/30.
At the moment I rent my longer lenses (Headshots, Ottawa and Toronto) so I don't have one at hand to experiment with, but will have to do my own experiments to see what vibration exists on the 400D/XTi.
When you saw vibration using the 100-400 after the exposure, there are likely differences in mirror dampening in each direction; not much need to dampen the mirror when it descends so that may have caused the movement you saw afterwards. OTOH, not much need to descend it rapidly, so it could be a softer impact.
The lightweight tripod should have poorer performance, implied by the poorer results in the PT article when the center column was extended. For dampening effect, a sandbag is sometimes hung from the tripod - perhaps for maximum dampening, perhaps it should actually be gently draped over the top of the camera body and lens?!
Another way to measure the movement of the lens would be to tape a small mirror or piece of aluminum foil to the end of the lens barrel, tape a laser pointer so it's always on, aim at the mirror so it reflects to the ceiling, and watch any deflection of the spot when the shutter is released. I may set that up.