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I just got the Lumix Nocticron 42.5mm lens and in the manual on page 7 it reads:

The aperture can be adjusted with the aperture ring only when a Panasonic digital camera is attached to the lens. (As of January 2014)

So, if I am using this lens on an Olympus OM-D body then does this mean that the lens behaves as though the aperture is set to "A" no matter what its actual position is? If so, why would this be? I would think that the aperture ring simply mechanically rotates the diaphragm. Why would it not work on non-Panasonic body?

I did test this by holding the unmounted lens up to a light and then rotating the aperture ring and sure enough the aperture did not change, so I guess it is true; but why would they do this? Seems like it would be simpler just to let the ring control the aperture. Why are they gimping the lens with this restriction?

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I did test this by holding the unmounted lens up to a light and then rotating the aperture ring and sure enough the aperture did not change, so I guess it is true; but why would they do this?

Why?, because the aperture is controlled electronically from the camera. The ring on the lens is basically an electrical switch that sends a signal to the camera body and has no direct connection to the aperture diaphragm in the lens. It's a pure 'fly-by-wire' control interface.

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  • \$\begingroup\$ The aperture ring gives you the traditional aperture ring interface, but as Michael says, it's just an elaborate electronic control. The reason that a manufacturer doesn't want the lens to stop down when you turn the ring is so that metering, composing, focusing, etc all happen with a wide-open, bright image. The lens stops down to the selected aperture when you press the shutter release button. This is basically how all electronic cameras have operated since the mid 1980s or so. \$\endgroup\$
    – osullic
    Sep 16, 2017 at 11:16
  • \$\begingroup\$ @osullic not universally true of mirrorless designs, given they can compensate in the viewfinder. For example, Sony in A/M modes. While you'd get a noisy EVF if you were stopped down hard in very low light - why would you stop down hard in low light unless on a tripod, in which case you have the time to manually handle your aperture. \$\endgroup\$
    – rackandboneman
    Nov 14, 2019 at 20:38

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