To reduce the aperture of a camera lens. If your pictures are coming out too bright, be sure to stop down the lens.Stopping the lens down allows you to focus on a single point in the frame, rather than having every part of the picture in focus equally.
stop down
v.
To reduce the aperture of some lens: The photographer stopped the lens down in order to increase the picture's depth of field. If your photograph is too bright, you should stop down the aperture.