To perform this test, I merely took 8 pictures in a row, handheld, with a Canon EF 300mm 4/L IS USM lens coupled with a Canon EF 1.4X extender. The first four were taken with the IS on and the second 4 with the IS off. The shutter speeds were varied from 1/320 to 1/40 sec for each setting. I tried to hold the camera as steady as possible for all shots. I shot the pictures with a Canon 30D in raw with sharpness set at 0. The images were converted with Canon DPP, then cropped to 100% with no other editing. I notice a few interesting things here. First of all, I got good images without IS at 1/320 and 1/160 second, which is already 1 and 2 stops, respectively, below the 1/focal length rule (this combo crops out to 670mm on the 30D). Secondly, not all of the images taken with IS are completely sharp. The one at 1/160 second, in particular, is a little out of focus. And finally, I was able to get reasonable images at 2 stops slower with the IS engaged. I use this lens/extender combination this way all the time with pretty consistent results, so this is no surprise to me.