Actually the camera doesn't need to know the maximum aperture of the
lens. It only needs to know the current amount of light entering the
lens at maximum aperture, and the offset from maximum. It's a very
simple system. No K or M mount lens has any provision for telling the
camera it's maximum aperture.
Godfrey DiGiorgi wrote:
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
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