On my Sigma 17-70mm f/2,8-4 DC MAKRO OS HSM, I have the impression that the image stabilizer (OS) is broken. If I switch it on, I can see no difference when I look through the camera, the slight movements from me not holding the camera steady are visible in the viewfinder. To me it looks the same as when the OS is switched off.
This is completely different on my Canon EF 70-200mm f/2.8L IS USM, which I took as a comparison. There, I can easily see through the viewfinder whether the IS is on or off.
Is there a better way for me to test the working OS of my sigma, apart from looking through the viewfinder of the camera? Or is it possible that the OS is only activated when I press the shutter release?