3. Framing Arrangements

Design challenges

  • Arrangements to accommodate Standard and Super 8 mm film formats and their different film frame to sprocket hole layouts.
  • Reliable centring of the film frame in the film gate aperture.
  • Simple and minimal adjustment.

For Standard 8 mm film the sprocket hole is aligned with the gap between the film frames. For Super 8 mm film the sprocket hole is aligned with the centre line of the film frame.

The distance between sprocket holes and the sprocket hole height are different in each format. Framing is made by adjustment of the distance of the motor sledge from the fixed film gate. To accommodate all the difference between the two film formats, the Cam rider claw end position is set to be at the first place after the film gate centre line where the leading edge of the sprocket holes of both formats most nearly align. Only minimum framing adjustment is then needed to centre the film frame of either format in the film gate. This occurs at the sixth leading edge of the sprocket hole after the film gate centre line. The distance for the Standard 8 mm format. from the film gate centre line is 21.57mm and for the Super 8 mm format it is 21.7415 mm. The small difference of distance 0.1715 mm eases the design of the framing adjustment facility. Figure 6 and Photograph 13 illustrate the above.  A typical film roller mounting can be seen at the top of the picture.

This was how I derived the claw end position.


For Standard 8 mm format the sprocket hole separation and height are 3.81 mm and 1.27 mm . The distance from the film frame horizontal centre line to the leading edge of the first sprocket hole after the line is:

  • sprocket hole height/2 + sprocket hole separation/2 = 1.27/2 + 3.81/2 = 2.52 mm
  • Nearest sprocket hole leading edge alignment occurs after 5 more sprocket hole separations.
  • sprocket hole separation * 5 = 3.81 * 5 = 19.05 mm
  • Total distance from the film frame horizontal centre line = 2.52 + 19.05 = 21.57 mm

For Super 8 mm format the sprocket hole separation and height are 4.234 mm and 1.143 mm. The distance from the film frame horizontal centre line to the leading edge of the first sprocket hole after the line is:

  • sprocket hole height/2 = 1.143/2 = 0.5715 mm
  • Nearest sprocket hole leading edge alignment occurs after 5 more sprocket hole separations.
  • sprocket hole separation * 5 = 4.234 * 5 = 21.17 mm
  • Total distance from the film frame horizontal centre line = 0.5715 + 21.17 = 21.7415 mm

Photograph 14 shows the motor sledge and framing parts. Framing adjustment is made by first slackening the motor sledge fixing screws of the motor sledge assembly so that it can slide in its slots. The motor sledge is raised against gravity by clockwise turns of the lower nut then the upper nut. The cam is then rotated to the home position to pull the next film frame into the film gate and then the sledge is adjusted downwards to set the Claw end position for correct framing using both nuts. The position is locked by tightening the motor sledge screws. Frame adjustment is only required for initial set up or when changing film formats. Slackening the motor sledge fixing screws also allows insertion or removal of the shim required to adjust the height of the motor sledge. The shim raises the height of the motor sledge and hence the cam rider claw so the claw end enters the Standard 8 mm film sprocket hole more centrally within in its 1.829 mm width.

The centre line of the video camera must then be adjusted to coincide with that of the film gate for correct video framing. See Video Camera Alignment.

1. Sledge fixing screws 2. Upper adjustment nut 3. Lower adjustment nut 4. Cam
5. Sledge end plate 6. Fixed pillar    

 

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