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A tunable aperture stop based on optofluidic technology, which overcomes a number of limitations of comparable micromechanical alternatives, is presented. Relying on the high optical absorption of aqueous pigment dispersions, we demonstrate that optical stops of high contrast may be defined without the need for any movable parts. The highly flexible design is based on photolithographic patterning of dry film resist and allows control of laminar flow in microfluidic chambers exclusively by capillary forces. The functionality of the developed device is shown in transmission measurements and by application of the iris in a basic imaging setup.