During the shooting of Day Watch they had a problem with the 'other world' sequences. These were shot in low light and focus was a real problem, quite regularly it went quite a ways off the mark.
Solution was simple, when the original shot goes out of focus composite in clouds of in focus mosquitos.

The trick is further enhanced by a few lines of dialogue along the lines of:
"Did you hear them when you were in there?"
"Hear what?"
"The mosquitos, didn't you hear them?"

Excellent movie too, trailor for Day Watch:


Bob.