I've read a few times now that the IS Version of the Canon L series lens can cause problems when focusing on stars and that a non-IS version is better - why is this? Price difference aside, if the IS is a problem then can't it simply be turned off on the lens or is it best to purchase a non-IS lens?
It is really about the 'park' position of the moving bits of optics when the IS is turned off. This is not a problem with my Canon 24 to 105mm F4L.
There are tests around of super telephotos 300mm and 400mm F2.8L both IS that show this problem of a slightly misaligned optical train that also shifts with an astro mounts orientation. The effect is slight 'coma' over the whole field.
Thanks Bert for the extra info. I'll have to do further research. I've had my eye on the canon 24-105mm f/4 L IS or non-IS (so perhaps no issues with the IS version if you and others haven't experienced problems), the 70-200mm f/4L (IS or non-IS) or alternatively the 70-300mm f/4L (IS or non-IS). I here the 70-200mm is a great astro performer for a zoom.
The 24-105mm f/4L IS USM is not available in a non-IS variant.
I love mine; despite it lacking microcontrast (meh, and, stuff) it makes for a spectacular portrait lens. Mine is one of the sharpest tools in the shed.
