| how
it works
shooting
film
manipulation
darkroom
techniques |
|
SX-70
operating sequence (how it works)
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- light path
- filmpack
- viewing mirror
- patterned mirror
- viewfinder mirror
- viewfinder lens
|
| 1)
Viewing With the camera opened up, all of its obvious
lines are at different angles. At this stage the film pack
(2) is protected from light by two mirrors. The image formed
by the lens is projected first on to the viewing mirror(3),
and reflected down onto a second, patterned mirror (4). This
acts as a flat condenser lens, concentrating the light rays
and and reflecting them up again to the first, viewing, mirror,
but at a different angle. From here they pass through a small
opening to the final, concave, viewfinder mirror (5) , which
projects the image through the viewfinder lens(6) to the photographer's
eye. Being bounced off the mirrors an even number of times---four---the
image is viewed right-way-round.
For focusing,
only the front element of the lens moves, requiring no mechanical
changes inside the camera. The illustration shows the basic
Alpha model, without the sonar autofocus; here a knurled wheel
engages the lens. With autofocusing, a transducer senses the
distance by the time-lag between ultrasonic pulses and their
echoes from the subject as soon as the shutter release is
half-pressed; a servomotor then focuses the lens. |
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- light path
- motor
- film
|
2)
Shooting As the shutter-release is fully pressed, the shutter/aperture
blades close to darken the inside of the camera. The patterned
mirror is then raised by a motor (2) to reveal the top sheet
of film (3); on the back of this mirror is another mirror, now
ready to reflect the image straight onto the emulsion. Being
reflected just once the image is reversed, but as the print
will be both exposed and viewed from the side, the effect will
be normal. These actions take about 3/10 second---what at first
seems to be a disconcerting delay in shooting.
The next step is when the twin shutter/aperture blades begin
to open. Each blade contains a circular opening with a small
kink in one side; as they slide across each other they create
an aperture that increases in size. A smaller pair of openings
cross to allow light to reach a meter, and when this senses
that half the required exposure has been made it signals
the closing of the blades. |
 |
- 'pick'
- rollers
- unexposed film
- exposed film
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| 3)
Processing Once the shutter has closed, the same motor
that lifted the base rear mirror now turns gears to slide
a 'pick' (1) that engages one rear corner of the film pack
and pushes it forward towards the rollers(2). The rollers,
themselves run by gears, take hold of the film like an old-fashioned
laundry mangle, and eject it through the front of the camera;
in the process they rupture the film's pod and spread its
contents evenly between the negative and the image-receiving
layer. The processing sequence is completed outside the camera(4).
Back inside, the base mirror has fallen back into position,
a spring in the filmpack pushes the next sheet up into position(3),
and the shutter blades are open once again for viewing. |
© Michael Freeman,
1985, Instant Film Photography.
It's SX-70 © Copyright
Joy M. Opfer, 1999. All rights reserved.
Legal disclaimer: I don't have a thing to do with Polaroid Corp.,
and they do not guarantee the accuracy of information of this site.
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