General Operation of A 35 Mm SLR
- Some lenses had manual diaphragms—the photographer had to take the camera down from his eye and look at the aperture ring to set it.
- A "pre-set" diaphragm had two aperture rings next to each other: one could be set in advance to the aperture needed for the picture while the other ring controlled the diaphragm directly. Turning the second ring all the way clockwise gave full aperture; turn it all the way counterclockwise gave the preset shooting aperture, speeding up the process. Such lenses were commonly made into the 1960s.
- A lens with an "automatic" diaphragm allows the photographer to forget about closing the diaphragm to shooting aperture; such diaphragms have been taken for granted for decades. Usually this means a pin or lever on the back of the lens is pushed or released by a part of the shutter release mechanism in the camera body; the external automatic diaphragms on lenses for Exakta and Miranda cameras were the exception to that. Some lenses had "semi-automatic" diaphragms that closed to shooting aperture like an automatic diaphragm but had to be re-opened manually with a flip of a ring on the lens.
When the shutter release is pressed the mirror flips up against the viewing screen, the diaphragm closes down (if automatic), the shutter opens and closes, the mirror returns to its 45-degree viewing position (on most or all 35 mm SLRs made since 1970) and the automatic diaphragm re-opens to full aperture.(See http://www.cameraquest.com/zunow.htm)
Most but not all SLRs had shutters behind the mirror, next to the film; if the shutter was in or immediately behind the lens it had to be open before the photographer clicked the shutter and then had to close, then open, then close.
Read more about this topic: History Of The Single-lens Reflex Camera
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