Using Masks for an Iris Effect

Now, for the last step: click on the timeline between the first and last frames on your mask layer, and then click the Tween dropdown on the Properties Panel. Select Shape to insert a shape tween; you should see the timeline turn pastel green with an arrow pointing from the first frame to the last. You should be able to drag along the timeline to watch the shape expand or shrink, then lock both layers to see how the irising mask will actually look.

Example.