This is the photo of the lens mount of an F-mount Tokina 10-17 mm (taken from online). What is the lever that I marked in red for? Note that this is not the aperture lever (which you can also see on the bottom right).

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The extra lever is the "Maximum Aperture Indexing Lever" or "Lens Speed Indexing Lever". It was added on some AI-S lenses to transmit the maximum aperture information to the camera body.

It was only used on FA & F4 camera bodies for more accurate automatic multi-pattern metering.

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    ...and Tokina appears to have chosen to use a metal tab on their F mounts that can be adjusted to suit different lenses in their lineup, rather than die casting a different rigid mount for each lens type. An economical design decision. – HamishKL Feb 24 '16 at 23:32

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