Hello:
Bracket the exposure.
In other words, Try 3 different F-Stops.
If your familiar With masks, You can use the Curtis Combo system.
It mounts on the lens barrel. 1 part of the mask put into a slot reduces half the exposure of part of the frame.
I don't know if Jack Curtis is still in business but here's his old address:
Jack Curtis 2231 Virginia Ave
Santa Monica, California 90404
You can also use a search engine. Search for the Jack Curtis Combo and advanced mask system