Originally posted by RKKS08 To use the Auto setting of the lens you need a camera which can control the aperture of M42 lenses. I think, with Pentax these were only the last 2 or 3 M42 models, before the switch to K mount. I think, the first Spotmatics couldn't use it.
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I believe you and a couple of other commenters are confusing a few terms that apply to M42 lenses and lens aperture that are related, but quite distinct. At the risk of appearing pedantic, here are the bullet points...
- Aperture automation (auto-aperture) historically means automatic aperture actuation (lens stops down to take the photo) via some sort of coupling (e.g. the silver pin on the lens rear). The vast majority of M42 lenses offer auto-aperture and on most, the function of the pin is switchable using a slider or switch on the side of the lens (the A/M or Auto/Manual switch). When an M42 lens says "Auto" on the front, that is what it means. Almost all SLR lenses (all makers) made since the mid-1960s are auto aperture.*
- Exposure automation means the camera will automatically set shutter speed and/or aperture based on the reading from the camera's meter. All M42 lenses having provision for manual aperture operation support exposure automation in Av mode. No M42 lens supports Tv or programmed exposure automation.
- Automatic aperture control means the camera is able to control the aperture opening. No M42 lens supports native aperture control as a feature.
- Open aperture metering means that the camera is able to meter with the lens wide open. No adapted M42 lens supports this feature. Pentax M42 body support is limited to the Spotmatic F, Electrospotmatic, and ES/ESII when coupled with S-M-C or SMC-series Takumar lenses.**
By context, I believe the OP was referring to the first term, auto aperture actuation.
Steve
* Along with auto aperture are fully manual , semi-auto (must be manually reset to open), and preset aperture. The last operates a little like the A/M switch in that the aperture ring is "pre-set" to a stop and stopped down using a second ring or switch just before exposure.
** It is interesting to note that the Chinon CEII and CE3 cameras provided both Av mode exposure automation AND open-aperture metering with ANY M42 lens. The feature was made possible by metering in real time as the lens stopped down.
Last edited by stevebrot; 08-05-2017 at 04:02 PM.