While still in the mob, shutting down after initial NVG FHT, I mention to very senior QHI that the goggles were not the best I'd used and the view was quite grainy. After laughing for what seemed like an eternity he told me they would have performed much better if I'd taken the outer lens covers off!
I passed the FHT.
For those wondering, the lens covers had a tiny hole centre for daytime viewing. Obviously big enough for my (then) young eyes.