The STF uses two iris systems for different purposes.

The manual stepless aperture is designed to stay as circular as possible. That matters because this lens is built to produce especially smooth out-of-focus rendering, and a round opening helps keep blur highlights circular instead of polygon-shaped.

The automatic aperture is the normal camera-controlled iris. It’s there for regular aperture control and for stopping down further when needed.

Why both? A highly circular iris is ideal for bokeh, but that design may not work as well when closing down to very small apertures. So Sony added a second aperture: one optimized for smooth blur rendering at wider settings, and another for practical camera-controlled stopping down.

In short: the twin-aperture system lets the lens preserve its signature smooth bokeh while still offering normal aperture operation and smaller f-stop settings when required.