Polaroid SX70 manual Fresnel screen

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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

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Contents Page Page General Description DescriptionOpening the Camera Camera OperationStarting the Automatic Mechanism Loading the Film packViewing and focusing Setting the exposure counterViewing and shooting Developing the print Exposures using flashCompleting the film pack Closing the camera Shutter Description Ambient Light Mode11 a Location & Arrangement of internal parts Shutter Description Flash Mode Page 14 S5 being opened by the recock ram Gear train Page Introduction Power SourceLens Viewing and Exposing SystemViewfinder optics exposing Fresnel Figures 2-3 a and 2-3 BFresnel screen Mirrors ViewfinderFlare baffle Camera Elctronics 1. GeneralSX-70 Switch locations Switch functionsS3 Being opened by recock ram S4 Switch operating configurations Function Sequence Switch Function TablePage Flash Timing Sequence Chart Shutter, exploded view Shutter Mechanical Details FigureA Solenoid S2 operation Sequence of Operations Film Processing ComponentsPage Max Introduction Initial Tests III TroubleshootingSX-70 Cmaera Repair Inspection Procedure Test Equipment Operation Power supply and pack simulator FigureLocation of new collimator target Collimator FigureDisengaging the idler gear Comprehensive tester Comprehensive Testers -6Model B Camera tester Model B Camera Tester Light Leak Tester Figure Comparalumen Light Levels TableLight leak test system Flash Simulator Board Figure10 Manual camera tester Manual Function Tester FigureManual Function Test Sequence Exit Pupil Test Optical System13 Aligning the exit pupil gage Entrance Pupil TestConcave Mirror Distortion Check USE of Analysis ChartsFresnel Quality Check Collimation16 Fresnel standards chart 18 Flex circuit terminal locations Camera/Shutter Isolation19 Test points on SX-70 shutters Camera Analysis ChartsPage Removal Front COVER/SPREAD Roll AssemblyInstallation Front cover hinge studs Repairs / Adjsutments Loose Front CoversBending the stop latch Replacing the Light Shield Cam Interference Between Front and Bottom CoversTop Spread Roll Looseness Front cover light shield cam Mirror Cover / Boot AssemblyBoot retaining screws