Sometimes I put my viewfinder to my eye before I turn on my camera. When I turn it on, the viewfinder image slightly shakes and moves a bit vertically. This also happens when I look through my viewfinder and press the shutter when the camera has been on, but not used for several minutes.
My camera is a 8 month old Canon 550D and I use it with a Sigma 18-50 mm f/2.8-4.5 DC OS HSM lens.
As it happens sometimes, the behaviour is hard to reproduce and hence I can not give you more information about the problem.
Does anybody know why this is happening?