Originally posted by Mannj What does the A/M switch do?
Auto/Manual
The word "Auto" in a lens name refers to having an automatic aperture. It does not have anything to do with exposure automation or auto focus. Automatic aperture M42 screw mount lenses usually have a control on the side of the lens barrel to a switch between automatic and manual aperture.
When on "Auto" and mounted on a M42 body supporting that feature (most do), the lens aperture stays fully open until the just before the shutter opens and then automatically stops down for the exposure and automatically opens back up immediately afterwards.
When on "Auto" and mounted on a non-supporting body (all K-mount cameras with adapter), the lens stays wide open regardless of whether an exposure is being made.
When on "Manual", the lens aperture is always stopped down regardless of what body it is mounted on.
When shooting with an adapter on a K-mount body, the lens must be on the "Manual" setting at exposure side in order to expose using the aperture set on the aperture ring.
As noted above, not all auto-aperture M42 lenses have the A/M switch. The most common being various Russian lenses including the M42 Zenitar 16/2.8 fisheye and Helios 44M-x 58/2. Both of these lenses require surgical modification before they have a functional aperture on a K-mount body.
Steve