1. A variable imaging mask for delineating the composition of a visible area comprising in combination, a sheath composed of a mask face and mask back, secured in opposition by guide rails, said guide rails having shoulders projecting towards a longitudinal centerline of said sheath, said shoulders abutting against respective shoulders of a non-separable moveable slide projecting in the opposite direction of the former shoulders when said slide is at a rear end position located near said closed end, said guide rails being mounted to said mask face and mask back by a suitable adhesive, with said sheath having one open end and one closed end and containing a rectangular aperture of a specific ratio of side length to height of a convenient size passing through said sheath with diagonally opposite and equal angle corners located on said centerline, said sheath enveloping said slide, featuring a v-shaped cutout, interposed within said mask face and mask back capable of altering said aperture size from fully open, through an infinite number of intermediate positions, to fully closed, whereby proportional views of said visible area corresponding exactly to the view yielded by an optical, light amplifying or electro-magnetic imaging apparatus at the same viewer to subject distance may be continuously viewed.