Canon Image Stabilizer makes my pics BLURRY??!!

Back yard at sunset

Jobsite at sunrise

The above images don't have anything to do with the post title. That's below:

My new Canon XSi DSLR camera came with an EF-S IS lens. The IS stands for Image Stabilizer. It has a mechanism built in to negate camera shake. I noticed that my first longer-exposure, low light shots had some blurring in them. This specifically showed up while shooting on the tripod. I figued my tripod was crappy and started pricing more solid versions. But then I noticed an odd effect whilst shooting the wine glass on the tripod last night. The image moved around in the view finder, noticeably, while the camer and tripod were rock steady. I then turned the IS off (there's a small switch on the lens.) The image in the viewfinder became still.

Strange.

To confirm this, I shot four images back to back tonight. I zoomed the lense all the way out (55mm) then shot with and without the IS. I also repeated the process using the mirror lock. The internal mirror is what allows you to see through the lens when looking in the viewfinder. It snaps out of the way when you take the picture, causing the loud click most people associate with taking a picture. Locking the mirror makes taking a shot a 2 step process. The first button push flips the mirror up. You let the camera settle, then press the button again to take the picture. This avoids any blur from the vibration of the mirror moving.

So, here's the result:

Details: shot on A-DEP profile with center weighted average metering and auto focus. A wired remote trigger was used. All were shot off a tripod with all legs in shortest position sitting on a table. All shots were taken within 5 minutes. Interestingly, the auto exposure, auto focus didn't choose the same shutter speed or f-stop on any of the photos. Before you cry foul, the sharpest shot was actually the longest at 15 seconds. The shortest shot was the no mirror, no IS image at 8.5 seconds. I don't know if the IS function in the lens affects the automated choice of shutter and aperture. Even if it does, it seems like it should make choices that minimize blur when the IS is ON, and that's not what shows up here.

So, if I'm shooting on a tripod the IS will definitely be turned off. Maybe it does help when hand holding the camera, I'll have to test that to find out.