An apparatus for alternating between two field frames in a camera viewfinder. The mask has a foldable mask with an aperture, with the mask being movable between two positions. In the first position, the mask is unfolded and interposed in the light-path of the viewfinder, thereby framing the viewfinder image in the shape of the aperture. In the second position, the mask is folded and stored compactly alongside the viewfinder, thereby not interfering with the image in the viewfinder.