Depth of field preview button
From The Glossary of Digital Photography
A button or lever on a camera that, when activated, causes the lens to stop down to the chosen aperture so that the photographer can see which subjects are in focus through the viewfinder. If the aperture chosen is not largest (smallest numerically), then the viewfinder will grow noticeably darker. By adjusting the aperture and holding the depth of field preview button or lever, the photographer can see how the depth of field will change. See also depth of field
