I've been making a series of long (4-10 second) exposures with the aid of a Heliopan ND filter with my newish 5D and 24-105L set on a tripod with IS activated. I got unsharp exposures. Should I be surprised? I tried again with IS switched off, and whilst I haven't yet had the opportunity to load these onto my computer, the results look better. This leads me towards a conclusion that IS actively makes sharpness worse with the camera on a tripod? Is this correct? The handbook, such as it is, indicates only that IS should be switched off to preserve battery power and makes no reference to reduced sharpness.