On Fri, Sep 11, 2009 at 11:17 AM, John Sessoms <[email protected]> wrote: > This seems to be an old inside joke on PDML, but I still don't understand > what an aperture simulator is supposed to do.
It's not a simulator. SLR cameras since the early 1960s have what is called an automatic diaphragm. What this means is that the iris in the lens is held open for best focusing and framing illumination in the viewfinder. At the time you release the shutter, the iris is automatically closed down to the taking aperture set on the lens, the mirror flips up, the exposure is made, then the mirror and the iris both return to the normal viewing/focusing mode. Then came coupled, "through the lens" (TTL) metering in the middle-late 1960s. Coupled means that adjustments to the exposure time and aperture setting immediately can be made to match what the meter is reading as the appropriate amount of exposure for the light in the scene. The older Pentax thread mount auto-diaphram lenses didn't have any way for the body (and meter) to know what aperture was actually set on the lens so the original implementation of TTL metering on Pentax bodies had a switch which mechanically stopped the lens down and turned on the meter. This meant that the meter would read the actual amount of light that would fall on the film at the time of exposure, with the lens stopped down, and of course knew what the exposure time setting was through the position of the shutter speed selector. While simple and elegant, this scheme meant some limitations on metering range and a certain amount of inconvenience to the photographer since the viewfinder would go dark while metering was going on. The solution was open-aperture TTL metering. Open-aperture TTL metering requires more information be exchanged between body and lens. The body's meter must know both a) the maximum aperture of the lens and b) the position of the aperture ring in order to calculate the correct settings based on the light coming through the lens. This was implemented in the form of a mechanical follow cam and lever setup on the lens and body (maybe two ... I'm not sure about the absolute details here) that transferred this information from lens to body so that the metering system in the body would know what the light falling on the film *would be* at the time of exposure, when the auto-iris was stopped down. This mechanical information passing system ... a follow finger mechanically driven by the aperture ring and a following finger it coupled with in the body ... was in place in all bodies and lenses made from the introduction of the K-mount in the early 1970s until somewhere in the late 1990s/early 2000s. I don't recall the specifics of which bodies did what, but the follow cam system started to become obsolete with the introduction of the A series lenses and on certain bodies. What happens with those bodies is that the aperture ring is locked to the A position, which turns on electrical connection to the body. The body then knows the lens' maximum aperture and the iris is controlled mechanically by the body moving the iris actuating lever rather than through the system of dead-stops provided by the aperture ring position and the auto-iris mechanism. With the introduction of even more sophisticated lenses with even more electronic information passing to the body, the in-body follow cam and in-lens aperture finger were removed. This simplification reduces manufacturing cost and inventory costs. All the information regards lens aperture, both wide open and range, is provided to the body via electronics, the body still regulates the iris by physical movement of the iris actuating lever directly. Removing the follow cam system from the body has the side effect of eliminating information passing from lens to body regards maximum aperture and aperture ring position for all lenses older than the A-series. Without this information, lenses prior to the A-series cannot support open-aperture TTL metering. > > Yeah, it simulates the aperture, but to what purpose? > > Can someone explain it in REAL SIMPLE terms? > > For reference, I have the following Pentax cameras to use as examples I can > look at it while I'm reading the explanation if any applies: > > K10D > *ist-D > PZ-1P > LX > Super Program > K1000 > Auto-110 Super > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. > -- Godfrey godfreydigiorgi.posterous.com -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
