Marcus Brooks writes:
The trick is to defeat the little hook thingy that shoves the film into
the rollers. See the notch on the back left corner of the cassette? Cut a
strip of thin steel that just slides into that notch on top of the film
(maybe 3/16 wide and 1.5 inches long), and bend it into a cross-section
something like the following:
_________________________ \ \ |
I cut the strip out of the lifter "spring" from an old cassette. I guess I made the "ramp" angle about 45 degrees. Its height should match the depth of the notch in the back of the cassette, so the vertical skirt is restrained by the back wall. (It's easier to do than to describe.)
You start with the cassette already loaded in the camera. In total darkness, pull the cassette back out of the camera and slide this device into the slot, with the bent part down and snugly against the back of the cassette. Then put the cassette back in the camera. Instead of ejecting the top sheet as usual, the hook should now ride up over the ramp and slip along the top.
Now take as many exposures as you want. When you're done, go back in the dark, pull the cassette out, pull out the strip of metal, and pop the cassette back in the camera. The top sheet now ejects normally, developing your multiple exposure.
Why would anybody go to all this trouble? Well, in my case I was working with a 3D ray tracing program back in the late '80s. I wanted a quick way to view the program's 24-bit output, but the company hadn't gotten any 24-bit displays yet. So I saved a separate 8-bit indexed file for each of the Red, Green, and Blue channels, hacked the color look-up tables to display linear red, green, or blue, and "photo composited" on the SX-70 by displaying each channel in turn and doing a multiple exposure.
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