Nikon Df - Manual (Page 187)

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Non-AI Lenses
The camera can be used with non-AI lenses (i.e., lenses not
equipped with AI mechanisms). Before attaching the lens, be sure
to raise the metering coupling lever; failure to observe this
precaution could result in damage to the camera or lens.
❚❚AI Versus Non-AI Lenses
“Non-CPU” lenses are divided into lenses with AI mechanisms (“AI
lenses”) and those without (“non-AI lenses”).
A
AI Lenses
Lower the metering coupling lever before attaching AI lenses.
Metering coupling lever
AI lens Non-AI lens
Meter coupling ridge No meter coupling ridge