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.
Paul
On 05/04/2009, at 5:32 AM, Godfrey DiGiorgi wrote:
I found the same thing ... The green button metering is accurate
wide open and near that, but accuracy becomes increasingly off as
the lens stops down.
Several possible causes come to mind, but the unfortunate truth is
that I never found a way to solve it that didn't have less
desireable side effects.
Godfrey
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