Attachable Exposure Meter This meter adds the convenience and speed of built-in exposure control to the Nikon F with standard prism or waist-level finder. It couples to both the shutter speed selector and the diaphragm of Auto-Nikkor lenses. You simply rotate the lens-aperture ring or speed selector until the meter needle is bracketed by either the gold or silver forked indicator. The meter has two sensitivity ranges (represented by the two forked indicators), and automatically shifts from high to low, or vice versa, depending upon light conditions. It covers film speeds from ASA 6 to 4000. The second version of this meter has a long window and no provision for a booster..