Lightleak or vignetting or sunshine

[wa1vgb]

I am going to go with too much shift. The snow in the picture is white, so the image is displayed as a positive. Not enough exposure on the top.

[Ian Grant]

I am going to go with too much shift. The snow in the picture is white, so the image is displayed as a positive. Not enough exposure on the top.

These camera's don't allow enough shift (actually rise) to cause that kind of problem and the lenses cause total vignetting of the corners, as the OP says the sky tone isn't darker, just the tree.

These lenses can be very prone to flare because they are uncoated when you read books from that era they recommend not to shoot into the light.

Ian