I used a 6D exclusively for almost a year, and ended up upgrading to a 5DSR to address specific shortcomings of the 6D I encountered in some real-world scenarios, the majority of which have to do with the autofocus system (surprised, right?
)The improvements I got from the 5DSR's AF system apply to the 5D mk III as well.
What the 6D did just fine:
- Landscapes, cityscapes, day and night. Tripod, manual focus. I had no compelling reason to replace the 6D for this type of photography.
- Studio portraiture/fashion. Shooting stopped down to f/8-f/11, I got good results using the center AF point and recomposing.
- Still life, product photography, etc. As with landscape, using MF and live view, so no issues.
- Low-light and indoor photography. The 6D's center AF point continues to work much farther past sundown than any other camera I've owned. It can practically see in the dark and lock focus in scenarios where the 5DSR needs AF assist.
Where the 6D fell short:
- Shooting wide open with fast lenses. At f/1.8 and wider, recompose is not an option, and outer AF point mediocrity is an issue.
- Moving subjects. A sophisticated zone-type AF system like on the 7D2 and 5D3 gets more keepers for sure. Tracking with AI Servo with just the center point is not easy, especially keeping it on fast moving small creatures.
- Nailing critical focus in environmental portraiture and outdoor fashion editorial work. Lack of precision of the 6D's outer AF points combined with size of their sensor area gave me a lot of subtle focus misses, especially at wider apertures. 5DSR's "spot AF" can land focus on a subject's iris where the 6D would get the eyelashes, eyebrow, etc. With the 6D I had to take more shots, refocusing each time, to increase likelihood of a keeper, and resort to cheats like focusing on the outer corner of the eye or eyebrow deliberately.
More than anything else, the 5DS AF precision and spot AF feature has been worth the upgrade to me, for shooting people. I like to use fast lenses wide open, and it's nice to be able to have good cross type points across the frame that I can trust. 6D's outer points worked only acceptably in shots stopped down enough to hide small misses to critical focus, and when I wanted shallower DoF I often used the 6D in manual focus (with precision matte screen installed) or in live view zoomed in. That worked, but it's so much slower than just being able to use phase-detect AF points that work well.