On Wed, Sep 17, 2014 at 10:02:56PM -0400, Mark Roberts wrote: > Zos Xavius wrote: > > >Ricoh claims that the aperture mechanism used in the M/K series is not > >accurate. My experience with green button stop down metering tells me > >that there might be something to that given the liberal amounts of EV > >compensation I end up using with various lenses. Oddly wide open > >meters fine with the green button. Hmmmm. > > With A-series and later lenses the aperture lever movement is linear > with respect to aperture area; with the K and M series lenses the > movement is linear with respect to aperture *diameter*, which makes > them much more dodgy (and is why Pentax changed it in 1982).
I believe even that understates the problem. The earlier K/M lenses, like the 'automatic' screw-mount lenses before them, relied on the aperture ring on the lens setting the limit of aperture stop-down at the appropriate point - all the camera body had to do was to push the linkage as far as it would go. I'm fairly certain that the 'linear with respect to aperture diameter' wasn't a very tight specification, and that different lenses might require pushing to a different point to achieve the same aperture. The movement of the aperture simulator lever (for lens-to-camera communication of the selected aperture) was well-defined, but the camera-to-lens actuation could have almost any response curve without affecting the actual exposure; the only two sta It was only with the KA-mount that we started to get lenses with a well- defined response to fine positioning of the aperture actuating mechanism. -- 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.
