Back in the good old film-days, F3, F4 and F5 aperture information was relayed through Nikon's "ADR" (Aperture Direct Readout) which was a window at the top center of the viewfinder that got its information from a micro-prism that read the small numbers at the top of the mounted lens AI and Ais lenses aperture numbers.If I recall correctly it was on/off illuminated on F5,,,