To decrease the size of aperture in a lens, e.g., to stop down from f/3.5 to f/16. Increases depth of field, requires longer exposure.

Stopping down refers to a photographic technique that increases the depth of field by reducing the aperture of a camera. However, this comes at the expense of reducing the available light and results in dimmer images. Photographers can compensate for this by increasing the exposure time, using a photographic film with a higher ISO rating, or, in digital cameras, increasing the light sensitivity of the sensor.