It stems from the mechanical shutter not being completely "flush" with the sensor and some oblique rays illuminating "under" the shutter near the edges, this is

It stems from the mechanical shutter not being completely "flush" with the sensor and some oblique rays illuminating "under" the shutter near the edges, this is supposed to even out with two curtain shutters, but no light can get past the efcs. There are a couple threads on DPR, I found this real quick: https://www.dpreview.com/forums/post/59542514 https://www.dpreview.com/forums/post/59542514