Controlled by a button or switch on some cameras, this feature permits you to look at the scene in the viewfinder with the aperture stopped down to the opening you intend to use when taking the pictur [..]

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 switch on some cameras that permits you to look at the scene with the aperture stopped down to the proper opening intended for the photograph.