The SX-70 is a bellows single-lens reflex camera-but one with multiple mirrors that bounce light rays back and forth, bending the light path to extend it. The image is reflected four times before the black and white photographer sees it, right side up, for focusing and composition. The image is reflected only once for black and white picture taking. This allows the light path of the 4 1/2-inch focal length to fit into a smaller package than if the image were not reflected. In addition to ten negatives, the black and white film pack, which slides into the base, also contains a thin, flat battery that powers both the motor and the circuitry of the camera. Since a battery is built into each black and white film pack, a fresh power supply is available whenever the black and white film is changed.
The camera body is made of a metal skinned plastic in which copper, nickel and chrome are plated to a thickness of 7/1000 of an inch over a glass-filled plastic base. The resulting composite material has the strength of metal and more resiliency. In the SX-70 camera, the image in the viewfinder is blacked out during exposure, as in other SLR cameras. But all the other events during the moment of exposure are different. Light from the lens, instead of going directly to the black and white film, is reflected once and then hits the black and white film; in this way the light travels further inside the camera and provides a substantially larger black and white picture.
Before the black and white picture can be made, the shutter-which had been opened for focusing-must close so that the taking mirror can be raised into place. Only when the taking mirror is in place can the shutter begin to open to make the exposure. Assuming that there is no time exposure, the whole process takes about 3/10 second. Black and white photographers familiar with other SLRs, in which pressing of the shutter release and making of the exposure seem instantaneous, may find the SX-70 a little disconcerting at first: it must be held steady for a fraction of a second longer than usual, or blurred black and white pictures may result.
The shutter of the SX-70 camera has two thin metal blades, shown above in red and blue that slide by each other when an exposure is made. Holes cut into them overlap to form two sets of holes: one, a large, round set, for the aperture of the taking lens; the other, a smaller set, for the light meter lens. When the exposure switch is pressed, the blades close in 18/1000 of a second and then, when the taking mirror is in position, start to open again. As the blades open, the light meter begins to measure the light, sending this information to the camera's computer. The computer has been programmed to allow the aperture to open until half the amount of light needed for correct exposure has passed through the lens.
At this point the computer sends a signal to the solenoid to change the direction of the blades and close the aperture. The rest of the light needed to complete the exposure will thus pass through the lens in the time it takes the aperture to close completely.Now: One-Step Black and white photography
The Finished Black and white print - Unlike other Polaroid Land cameras, the SX-70 ejects the exposed black and white film automatically with a motor-driven gear and pick system; there is no need for the black and white film to be pulled out by hand. Nor is there any need to count seconds and separate the negative from the black and white print at the proper moment, for there is only enough chemical in the pod to develop the black and white picture perfectly; when the chemical is used up, development stops au¬tomatically. By then, the opaque dye that has kept light from spoiling the process fades away, and the black and white picture appears in color. Furthermore, there is no need to coat the black and white prints since the clear acetate top layer provides a permanent protective cover.