Collin Brendemuehl wrote:
It doesn't make mechanical sense
If two 50mm (K & FA, for the sake of argument) lenses
have the indicator set to f "8" and f "2" is the same
diameter, for the body to close the mechanism down to
f "8" would not the distance be identical?
On the original K-mount, moving the stop-down coupler closed the
aperture until it reached a mechanical stop, set by turning the aperture
ring. You could continue to move the coupler beyond this point but it
had no further effect on the aperture. Hence the relationship between
absolute coupler position and lens aperture could be undefined, because
all the body had to do was move the coupler through its full range to
ensure that the lens was set to the correct aperture.
With later K-mount versions (for A, F, FA and D-FA lenses), this mode of
aperture setting was maintained, but in addition, the aperture could be
controlled from the body by moving the mechanical stop out of the way
(by setting the lens to its smallest aperture) and only moving the
stop-down coupler part-way. In this scenario the relationship between
coupler position and lens aperture must be strictly defined, since
that's how the aperture is actually set.
FAJ and DA lenses lack the aperture ring and its mechanical stop
entirely and their aperture can only be controlled from the body.
Make sense?
S
