The NPC Proback II instant film backs use a free-floating fiber optic lens to transfer the image from the camera film plane to the Polaroid film plane without any degradation of the image. Because the fiber optic bundle is in complete contact with the Polaroid film, the image is transferred with absolute sharpness and fidelity. There is no light loss and no color shift. The Proback II lets you make two independent contact size images on each sheet of Polaroid film simply by pulling the white film tab to a predetermined position before taking the second picture. Instantly, you can do a side-by-side comparison to evaluate varying composition, subtle lighting changes, or results produced by lenses of different focal lengths. Manufactured with the highest quality materials, the light seal mechanism is composed of stainless steel and beryllium copper. All other interface components are black anodized aluminum and stainless steel. We believe that this is the best way to insure as good an image as possible.