Originally posted by arnold I am not sure about this, Your suggestion is stop-down metering, which is fine for the SP, but the SPF is designed for open metering with a lens that communicates aperture. My auto Tacumar has nothing on the back but a stop-down trip button, The camera won't know the lens has closed. Am I missing something here?
Perhaps
While the OP's lens has the feature, not all Auto-Taks are semi-automatic. Some of the later variants have fully automatic diaphragms as found in the Super-Taks. A good summary may be found on the lens database page for the screw mount 55/1.8 Takumars.
https://www.pentaxforums.com/lensreviews/SMC-S-M-C-Super-Auto-Takumar-55mm-F1.8.html
As for the SP F...Yes, it does support open-aperture metering, but only with S-M-C/SMC Takumars (most) and a few transitional ST lenses. With other M42 lenses, legacy stop-down metering is available via the stop-down/meter switch on the left side of the lens mount. When a legacy lens is mounted the switch works the same as on earlier model Spotmatics. This is covered on p21 of the user manual and calls out the Auto-Takumar series explicitly for this technique. The camera does not need to know if the lens is stopped down. If the stop-down meter switch is in the up position, the actuator/"flipper" in the mount opening will engage the pin on the lens to stop down the lens. The camera will meter based on the light reading at the sensor modified by the bias applied by the aperture simulator (in this case, none) .
Steve
Last edited by stevebrot; 09-06-2016 at 03:58 PM.