What you'll need
The traditional iris transition, common in the early days of cinema, provides an alternative to a fade-in or fade-out. Use this playful masking technique to draw the viewer’s attention creatively to something specific before the rest of a scene comes fully into view.
Starting with your footage in a composition, create a new solid (Layer > New > Solid) of any color at the top of the composition. Select the Ellipse tool in the main toolbar and draw over the area of the shot you want to mask, pressing Shift while doing so to confine the shape to a circle. Choose Layer > Transform > Center Anchor Point in Layer Content to center the anchor point in the selected mask.
To reveal the background footage through the mask, select the footage layer and choose the Alpha Matte option in the TrkMat column, targeting the solid above it. (If you don’t see TrkMat, toggle the Switches/Modes button at the bottom of the Timeline panel.)
After the footage shows through the circle mask, keyframe the mask’s Scale (S) from zero to however large it needs to be to achieve the iris-opening speed you want. Increase the Mask Feather setting (F), depending on how soft you want the edge of the mask to appear.
Reposition the mask so it opens exactly where you want it. Try adding extra Scale keyframes between the two extremes to finesse the iris-opening animation until you’re happy with the result.
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