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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.
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.
Another thing to note about the use of the aperture ring when using manual lenses on Pentax bodies is that you'll need to press the green button to "stop down" before taking your shot. This triggers the aperture lever and so when you take your shot the lens properly closes rather than stays fully open. This procedure is required on the K series lenses. Via this method, the camera will be able to sense the proper light levels and offer a shutter speed. Note that this will only work in Manual mode. If you use Av, then the aperture will always be fully open. On the older non-automatic lenses (those without the aperture lever like the screw-mount lenses), when you stop down the iris will close and remain closed.
