I was told when I did my initial medical over 20 years ago that photo chromatic lenses weren't allowed because of a heavy landing by a DC9 Captain. He had been flying in bright sunlight over 8/8 low cloud, when he descended the lenses didn't have enough time to clear before he touched down in a fairly dim environment, misjudging the landing because he couldn't see properly.
Are modern lenses better or is the prohibition still in effect ?