[stoppingdown]

The plot thickens. I got a new copy from Amazon (the former copy was bought from another reputable shop), and it shows the same behaviour. Chances that both lenses are from an hypothetical defective batch are very low.

Furthermore, new questions:

1. Why don't I see the problem in the EVF? Shots are at full aperture, so the diaphragm stays in the same position before and during the shot.
2. Running a test keeping ƒ/1.7 but changing the shutter time, the problem slightly decreases shortening the shutter time, until it goes out. So... maybe is it related to the shutter rather than the diaphragm? But how could that happen? Both the camera bodies I'm using have no problems at all with other lenses.

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Using the silent shutter of the a6600 (electronic only) the problem doesn't happen... So maybe is it really a strange interaction with the shutter?