TheAnt wrote in post #16339576
1. There will be a black circle. The image circle isn't big enough to display everything.
2. No. Don't use non-FF lenses on FF cameras. They will damage the mirror.
While in most situations I don't recommend using non-FF lenses on FF, neither of these statements above are absolutely true. It depends on the lens.
For instance, the Tokina 11-16 2.8 is a lens for crop sensor cameras but it will not damage the FF mirror and will not even vignette on full frame at 15-16mm (perhaps some very minor vignetting.) The same is true to the Sigma 18-35 1.8. At 35mm its image circle actually fills the FF sensor, making it a pretty nice 35mm 1.8 "prime" lens on FF. I also know that the my Sigma 10-20 does mount on my 6D and does not damage the mirror. So it cannot be universally declared that all crop lenses will damage FF mirrors.
Canon EF-S lenses are some that may cause damage, that's also why they cannot be physically mounted on FF cameras. Their rear elements protrude out more than third party crop lenses'. Third party manufacturers do mount FF cameras, and whether they damage the mirror probably varies but I'd say most of them do not. DISCLAIMER: don't take my word for it, and don't try this at home if you don't want to risk damaging your FF mirror.