Polaroid SX70 manual Fresnel Figures 2-3 a and 2-3 B, Viewfinder optics exposing

Figure 2-2 Viewfinder optics (exposing)

the subject passes through the camera lens and is reflected by the viewing mirror (on the inside surface of the mirror cover) to the Fresnel viewing screen.

The entire subject image is now reproduced on the viewing screen. At this point, viewfinder optics must be employed for focusing. The optics must transfer the image from the viewing screen to the eye (done by the parabolic mirror and the viewing mirror) and must present the image in a fairly sharp contrast to permit proper focusing (done by the eye lens and parabolic miror).

1. Fresnel (Figures 2-3 A and 2-3 B)

The special Fresnel (pronouced Freh-nell) screen is employed to enhance the viewing image by increasing the brightness and defintion. If a matte white surface were substituted for the Fresnel screen, an image would still be visible. However, the light rays striking nearest the corners of the screen meet the screen at a more oblique angle than the rays striking near the center. At this greater angle, a larger percentage of the light is dispersed and less light is returned to the viewer. Thus, on a matte-screen image the corners would appear darken than the center. The Fresnel screen is designed to overcome this problem.