In the short film Life Lessons by Martin Scorsese, the iris effect is used a few times in seemingly arbitrary places. But from watching the film a second time, after already knowing what it's about, I came to a conclusion that this effect is used to focus on things, places, and people where the protagonist, Lionel Dobie, is sharply attuned in his life. There is an iris effect when we see his paints, which is clearly a focal point in his life, and when we see him standing alone in his studio; symbolizing his narcissism -- which we see later in the film from his inability to tell Paulette, his assistant and love interest, simply that her work is "good," out of fear of tampering his artistic integrity.
Other places we see the iris effect is when Lionel see's Paulette for the first time after her trip when he picks her up in the airport. It centers in on her and becomes slow motion -- the time drawing out longer than it probably actually is. Paulette is one of Lionel's obsessions, just as his painting and his ego is. We see the iris effect once again on Paulette's foot, before Lionel says he feels a strong urge to kiss it. Not only does the iris effect represent his obsessions, but his obsessive impulses; painting being one of them. He feels an enslaving urge to paint when he feels repressed or convulsing with emotion. Then at the very end, we see the iris effect when he's at his opening gallery and meets a new women to whom he offers a job as his assistant. And thus begins the cycle all over again, arising from a brand new obsession.