Do they make tripod collars for the EF 100mm f/2.0 USM lens? I do astrophotography and stars in one half of the image have a green tint to them while stars in the other half have a purple tint to them - even at f/4.5. I want to see if it's caused by sagging.
Also, suppose it's not caused by sagging and I bought the lens over 3 months ago, will Canon fix this? Is it covered by warranty?