I don't know if it is clear from the above comments, but the word
Auto means automatic aperture stop-down at exposure time. It does not mean support for exposure automation or programmed exposure automation (the "A" position on some K-mount lenses). M42 lenses come in four flavors:
- Manual aperture -- the iris opening is always coupled with the aperture ring
- Preset aperture -- the iris opening is full open unless the preset ring or switch is moved to the stop-down position
- Semi-auto aperture -- The iris is open until a pin at the rear of the lens is depressed, at which point it stops down to the position set by the aperture ring. The iris is reopened with a lever to be ready for the next exposure.
- Auto aperture -- Like semi-auto except that the iris reopens by itself after the exposure
Auto and semi-auto aperture lenses often featured prominent labeling (e.g. the Auto-Rikenon 50/1.7 that came with my Singlex TLS). It was a huge convenience factor and a selling point back-in-the-day and by 1970 most lenses on the market featured auto-aperture. Almost all K-mount lenses have automatic apertures as do most M42 Asahi Takumars. A feature on most "Auto" M42 lenses is a switch or slider near the mount on the side of the lens that disengages the auto-aperture mechanism and puts the lens iris in fully manual mode. It is that switch that allows forward compatibility for adaptation to K-mount bodies.
The A/M switch must be in the "M" (manual) position at the time of exposure in order to use the lens other than wide open on a K-mount body. Some M42 lenses lack the A/M switch (e.g. Helios-44M-4) and require a little surgical modification to make them adaptable to K-mount. That is, unless you have one a very few very expensive adapters that have a flange to depress the aperture actuation pin. Metering and exposure mode support varies by camera body with a general rule that shutter-priority and program exposure automation are never supported.
Are we confused yet?
Here is a quick cheat sheet to determine if a lens will work on your K-mount body:
- No silver pin on the base? The lens will work.
- Silver pin and A/M switch? The lens will work.
- Silver pin and no A/M switch? The lens will not work without modification or special adapter.
Note that there are still a few incompatibilities based on physical projections (some later Mamiya/Sekor) or non-standard dimensions (some Helios-44-3), but for the most part the three bullet points are pretty dependable.
Steve
BTW...countrary to the advice in a post above, Av mode is not recommended for stop-down metering on the K-5. M-mode with green button will provide consistent stop-down meter accuracy with M42, Pentax-K, and Pentax-M series lenses. Av mode generally will not. Why this is so is a long story and is discussed elsewhere on this site.