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A mechanical lever or electronic button used to close the lens aperture down to the actual exposure setting so depth of field can be previewed.
A button or lever on the camera that either stops the lens aperture down manually or electronically. This enables you to view the image at the aperture that will be used to take the picture. The view will be darker, but you will be able to see exactly what will and won't be sharp or in focus. See "Using the depth of field button" in the Techniques section.