Originally posted by macman24054 
I have a rather stupid question. How does the camera stop down a M series lens?
Really short answer, with no contacts on the lens, in M mode, the aperture arm in the camera moves the lens's aperture lever from fully open to fully closed. Setting the ring on the lens limits how closed the aperture gets in fully closed position. In all other modes, the camera doesn't move the aperture arm at all with a no-contact lens.
Really long answer:
It's not stupid, it's the history of the K-mount!
In the M42 days, camera makers knew it would be handy to focus with the lens wide open but meter correctly by knowing where the aperture ring was set. They had a lot of solutions to this over time, got it right at the end, then everyone decided to switch to bayonet mounts. The K-mount would start with wide-open focus and metering that read the aperture ring position from the beginning. The new bayonet-mount lenses had an aperture lever that moved between wide open and stopped down to wherever the aperture ring was set. They included a small tab in the mount that moved with the ring. The new K-mount bodies would move the aperture lever to hold the lens wide open as it was mounted, then release it when a photo was taken. Another small arm would read the position of the lens aperture ring and use that in meter calculations. It all worked, the user just needed to buy all-new lenses for full compatibility, everyone was happy.
Shortly afterward, camera makers started thinking again. What if the camera was a bit smarter and could choose its own aperture and shutter speed? That would be great, except maybe it would be nice to retain compatibility with those "old" lenses just introduced less than 10 years ago. So they invented a new system where the aperture arm in the camera could sometimes move the lever a little bit at a time instead of just open-closed. They needed to make new lenses that told the camera what the aperture range was for that lens. The aperture lever mechanism in the lens had to move the blades exactly one stop when the lever moved a certain distance. And the aperture ring needed an A position to tell the camera body to use all this new technology. At that point (early 80s), they could have made camera bodies without the arm that read the aperture ring position, but there were still lenses in the catalog that needed it. So each new body could use lenses in two ways: as an original K-mount type where the aperture ring ruled all, or as an A-type where the camera body controlled the aperture. The new program modes needed the camera body intelligence but as long as everyone bought all-new lenses, everyone was happy.
There is more history but two key points: Pentax DSLRs have the "new" aperture arm type but no arm to read the aperture ring position. The only information they can read from a lens is through electrical contacts. Clever camera technology (multisegment metering, flash automation, all modes except M) requires precise aperture information. But in full-manual mode, you are not using a lot of that stuff anyway. All you need is the stuff from the original K-mount: wide open focusing and metering based on the lens aperture ring. Unfortunately the missing arm makes real-time metering impossible. So the Pentax designer solution is when a lens without contacts is mounted, the aperture control arm acts like the original K-mount arm, just open or closed. You can control that action with the green button, which also switches on the meter (which sneaks into center-weighted mode that's compatible with no lens data) . It's not quite as smart or integrated as a K1000 but it does work. It even works with no aperture lever, like an M42 lens.
Post #10 by robthebloke
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