From Canon's release, the differences between version II and I of the Canon EF 24-70 f/2.8L :
- Reduced CA at wide angle edges/corners
- High resolution and contrast in images, crisp clear edges around subjects (may or may not be an upgrade)
- Optimized lens coatings: color balance improvement, less ghosting, less spherical aberration.
- Circular nine-blade diaphragm (instead of 8-blade; probably rounded aperture blades as well)
- Improved build quality (more durable, better weathersealing)
- Smaller / less heavy
- 82mm filter (instead of 77mm)