A cool feature would be - like the first poster implied - a brief moment where the enviroment is visible and then fading out, simulating the kaleun closing the other eye and focusing trough the lens, another one would be the enviroment coming visible for brief moments when a lot of movement shakes the boat like a storm or explosions nearby simulating the kaleun losing focus for a brief time and/or stumbling slightly, thus losing his immediate contact with the lens with the open eye.
I know it would be very hard to simulate and make working properly, but it would make for a cool feeling and make the player more aware of the world outside the periscope... For myself i tend to focus so hard i sometimes firget that the periscope is actually attached to the uboat and not just a free-floating lens, haplessly adrift on the ocean heading towards a tasty tanker...
Naturally, it would be optional much like other realism-options.