The issue could be related to the way the diaphragm mechanism works in
interaction with the body's iris actuation mechanism, but the
linearity or non-linearity of the iris regulation mechanism doesn't
matter. When a K or M-series lens is on the camera and the camera is
in manual exposure mode, it stops the lens down accurately to the
setting on the aperture ring. I tested for that with a hand held
meter ... proper exposure is achieved with every apert...@time setting
when using a K or M-series lens in Manual exposure mode and the meter
reading is taken from the external meter.
Lenses like the FA77/1.8 work fine in manual metering mode when used
normally (lens locked on the A setting, aperture set by the body
controls) but show the same metering issues as K and M series lenses
when the aperture ring is used to set the lens opening and the green
button stop down metering is used. I tested that too.
The mechanical aperture follow-cam sensor of old would allow full
aperture metering with these lenses, but it doesn't affect how the
iris regulation mechanism works or how this metering mode functions.
Godfrey
On Apr 4, 2009, at 6:19 PM, Paul Ewins wrote:
AFAIK the diaphragm on the K & M lenses works differently to those
on the A, F & FA lenses. One is quite linear in action, the other
more of a curve. If the metering can't tell the difference between
the two then one will be accurate across the range and the other
will be accurate wide open and get increasingly innaccurate as you
stop down. I believe this is the essence of JCO's lengthy aperture
simulator (stimulator?) crusade as that was the missing link that
made both systems work on the same camera.
So a 77/1.8 will work fine in manual mode but a K85/1.8 or M85/2
will need you meter at full aperture and then count the number of
stops from wide open as you stop down and adjust exposure
accordingly (easier to do than say). On a SMCT 85/1.8 you simply set
the lens to manual and stop it down and meter. And I'm not sure what
happens on the M42 lenses that had the two pin system for the ES and
ESII.
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