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250mm lens repair article
After 3 screws are removed from around the outside of the aperture
assembly it will separate into two halves.
The top side mounts the bearings and is not shown , but we have here
the iris blades.
This is where the problem lay with this particular lens.
One of the blades at the bottom (or top) of the stack must have had a
slight twist which was causing it to bind up on the housing when fully
open.
This resistance was greater than the closing springs tension so it
stayed locked open instead. :-(
The solution was simply to reassemble the blades in a different order.
