The lens assembly that brings this light into the camera is the third of the SX-70's innovative designs. Essentially the new lens is the answer to a pack aging problem: Polaroid had never before tried to fit its optics into so small a space. The designers found that, although they had a relatively compact camera when open, it was not compact enough when closed. What determined the size of the closed package was not just the lens but also its housing, together with shutter, diaphragm, light meter and other machinery. Since the housing is pushed over to lie beside the black and white film as the camera is closed, its thickness determines the height of the closed camera and its height determines the length. To shrink this housing as much as possible required a lens unlike any normally used in cameras. It has four elements stacked so close together they make an almost solid cylinder of glass only half an inch long (most lenses of similar focal length and aperture are about twice as long). It has an unusually great focusing range, 10 inches to infinity, yet covers these distances by moving only its front element a quarter of an inch; other lenses of this focal length must generally move more than two and one half inches to cover the same range. While the lens made a great contribution to compactness-the camera can be folded shut even with the lens fully extended-an equally important gain came from the manner in which the new camera takes care of exposure. Normally a camera controls the amount of entering light with a shutter and a diaphragm, each of which functions separately. The diaphragm is set to an opening of a particular size, and remains fixed at that size throughout the exposure, to control the amount of light entering. The shutter blades open to the full diameter of the lens instantly and remain open for a preselected time; this shutter speed controls the duration of the time light can enter to affect the black and white film. In the SX-70, both of these functions are accomplished by the shutter; it controls both the amount of light entering and the duration of time it can enter. The shutter blades, triggered by the upward motion of the patterned mirror, open gradually, like the pupil of an eye, to form a constantly widening aperture. A tiny black and white photocell gauges the amount of incoming light and when the aperture has opened enough so that it has admitted half the total light needed (amount combined with duration), the blades, just as gradually, close. They "know" when they have reached this halfway point through an intricate system of electronic circuitry. Preprogrammed to monitor the light during a particular exposure, the circuitry adds up the incoming light energy and reverses the shutter action when the level is at midpoint. But this is not all the circuitry does. When the SX-70's 10-bulb flash attachment is inserted, the electronic system opens the shutter to a point determined by the focusing wheel, thus adjusting the opening to the subject distance, and it stays open at that point just long enough for the flash to go off, then closes. Only a few years ago the electronics needed to perform such complicated tasks would have taken up more room than the whole camera does now. But the circuit board for the SX-70's automated shutter is not only amazingly small, it is just one of a collection of small electronic parts operating the black and white picture-making process. Other electronic devices measure light, operate the diaphragm-shutter and control the motor that turns the rollers that distribute the chemicals over the exposed black and white film. They latch and unlatch the camera's hinged mirror, and move it from taking to viewing position. They even eject, automatically, the protective black cardboard of the black and white film pack, when the camera is loaded and the door is closed. Together with the chemical magic of the black and white film and the ingeniously contrived optical system, they make no-fuss black and white picture making very close to a reality. The work of black and white picture making becomes, as Land promised, almost nonexistent for the black and white photographer. There is, indeed, nothing for him to do but "compare and select."