Abstract: A portable viewing mask for delineating an image to be recorded or re-composed artistically, photographically or by electro-magnetic means having a variable size viewing aperture. The mask consists of a sheath containing a movable slide which adjusts the size of the viewing aperture. The aperture is positioned with diagonally opposite corners along the sheath's longitudinal centerline. The coincident longitudinal centerlines of the slide and mask sheath result in the maintenance of a constant aperture proportion as aperture size changes. The proportion of the aperture can be any desired ratio and size. In artistic use, the mask is held a determined distance from the viewer's eye and acts both to identify a pleasing composition for recording and to shield unwanted or distracting visual elements from view. In optical recording applications, when calibrated to the user's optics, it yields a field of view identical to the field of view of the appropriate lens seen from the same spot.