Because of the extreme extension of this lens, the aperture actuator rod has a long slot that engages a pin on the iris assembly.
This long section is flexible enough so that the pin can jump out of the slot causing aperture malfunction.
It can be fixed by bending the slotted portion slightly toward the outside of the lens but not so much that it binds on the lens.
I just use a long, thin screwdriver through the lens opening.
Note antireflection flocking applied to inside of lens and careful rear light baffling to reduce internal flaring.