The screen itself is a sheet of plastic upon which are impressed series of concentric rings much like the grooves pressed into a phonograph record (Figure 2-3) instead of having a vee shaping; however, the grooves in the Fresnel screen form a saw-tooth with a tooth angle increasing slightly with each successive groove in such a manner as to complement the decreasing angle of the light ray. The saw-tooth flattens out completely at the exact optical center of the screen. In the SX-70 camera, the optical center is not the geometrical center of the screen.
The surface of the screen is silvered to provide optimum reflectance. The result is a brilliant viewing image evenly illuminated from corner to corner.
For the reader who is familar with the use of a Fresnel lens used in conjunction with a ground- glass focusing screen, or a Fresnel-ground focusing magnifier, it must be noted that these are transmission devices while the Polaroid screen is a reflecting medium. Otherwise, the brilliance enhancing properties are the same. A suitable analogy would be the comparison of a lenticular projection screen compared with a matte surface. The ribbed surface of the lenticular screen narrows the angle of reflectance but
increases the brilliance of the image within teh viewing aera.
In summary, the Fresnel satisfies three conditons: 1 - it enhances focusing by distributing light rays evenly across the entire viewing aera.
2 - it guarantees proper focusing by nature of its acting as a ground glass - thus insuring that the subject is in focus when the eye sees the image in focus on the screen.
3- it acts as a reflecting surface to permit the eye to see the image through the viewfinder optics.
As show in Figures 2-3 A and 2-3 B, two types of fresnel screens are used. Current types (Figure 2-3 A) have a split-image circle cut into the center of the screen which makes focusing easier. Specially oriented prisms within the circle split the image unless the camera is perfected focused. Therefore, a straight vertical line in the image aera would appear to be broken unless the subject is in focus. To further ease focusing, the image within the split circle appears brighter than the rest of the Fresnel image. This is possible because the split circle is not coated in the same manner as the rest of the Fresnel. This latter feature improves the ability of the viewer to focus the camera in dimly lit aeras.
Figure 2-3 Fresnel screen
