I have taken this picture with an ES ED80, a Starfield 0.8x reducer/flattener with a Canon 100D (APS-C size sensor).
The flattener corrected the field curvature but introduced some coma-like aberrations on the corners. The specs of ED80 are within those recommended by the manufacturer, and the backfocus is 55mm, which is what I have with standard T-ring.
So is this to be expected (although the description says should correct the field on aps-c sized sensors). Can it be corrected by further fine tuning the backfocus?
Would love to know your experienced opinion, Thanks!