[davidmanze]

Magnesium is no better shield against high frequency - and in front of the plastic body is a metal bayonet. Also, the mirror box is not only plastic?

 

If I look at this explosion drawing, I see at least two parts potentially made of some cast or sheet metal, which are in between the sensor and the radiating HS drive of the Tamron - this could be enough to prevent banding.

 

Magnesium is a very good shield against high frequencies, that's why WiFi  antennas have to be behind a plastic body part, even though the rest of the body is magnesium.