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Message: <blockquote data-quote="Bob Blaylock" data-source="post: 601399" data-attributes="member: 16749">My D3200 has that too.  I think I know what it's for, but I am not certain.  That's very close to what I think.  I don't have any “D” lenses, I just have the one “G” lens that came with my D3200, and several very old, completely non-electronic lenses.  As you say, with electronic lenses, that are equipped with aperture rings, you're supposed to set the aperture ring to the smallest aperture, or the camera will give an error.  I believe the “button” in  [USER=27017]@cwgrizz[/USER]'s picture is to detect whether the aperture ring is set to its smallest size, and to allow the camera to give an error if it is not.  That lever is on nearly all F-mount lenses, all the way back to the very first ones in 1959.  Originally, it was to allow open-aperture viewing.  Regardless of how the aperture ring was set, the camera could use this lever to force the aperture to be open all the way, while looking through the viewfinder, and then release it to allow the aperture to close to the setting on the aperture ring when the picture is actually being taken.  The AI-S development, I think in the early 1980s, calibrated that lever so that with the aperture ring set to the smallest setting, the camera could use that lever to accurately set the aperture, making shutter-priority auto-exposure possible.  That's why, on cameras that use this lever for that purpose, the aperture ring needs to be set to the smallest aperture; otherwise, the camera can only set the aperture as small as where the ring is set.  This AI-S development predates lenses with any built-in electronics by over a decade, I believe.  “G” lenses eliminate the aperture ring entirely, and only allow the aperture to be set via this lever.</blockquote>

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