Can I use aperture ring of Pentax FA lens in aperture priority mode?

Question

I'm considering getting an FA Limited lens for my Pentax DSLR. The FA series has an aperture ring with an "A" setting for control by camera bodies that support it.

Can I instead use the aperture ring to set the actual aperture value in Av (aperture priority mode), letting the camera pick a shutter speed, or am I limited to the "A" aperture and have to use the thumb dials on the body to control aperture? I don't want to go full-manual mode.

Answer

To use aperture priority on Pentax dSLR with FA lens, you have to leave the aperture ring to "A" and select the aperture on body.

With aperture ring in A mode, the FA lens will behave just like a DA lens. You can use Av mode and set aperture on the body, shutter time will be calculated according to automatically measured exposure.

With aperture ring set on a specific aperture, the body does not know what you have set it to and would not be able to select shutter speed. By default, shutter will not work. You can enable using aperture ring in Custom Settings, but that will just enable using shutter - the lens will be used wide open regardless of the setting on aperture ring.

That's where the "crippled" part of the "crippled KAF2" mount on Pentax dSLR-s shows up - they do not have the mechanical linkage to read aperture settings from an aperture ring.

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