|External diaphragm plunger (see another view) of 58mm f/2.0|
Jena Biotar, aligned with shutter release of Exakta VX500
image by Süleyman Demir (Image rights)
In reference to lenses, PAD means "pressure activated diaphragm." It is also known as APD for "automatic preset diaphragm." This was a mechanism seen in some 1950s lenses for Exakta, Miranda, and others, where a small arm protruding from the lens piggybacks over the shutter release on the front of a camera body. This arm carries a button that when pressed closes down the lens diaphragm to the aperture selected on the f/stop ring; the button also transmits the finger pressure to the shutter release behind it, so that the iris is stopped down just before the shutter release is tripped.
With the 1959 appearance of the Nikon F and other Japanese cameras offering internally-coupled diaphragms, reopening instantly, the popularity of the PAD approach soon waned.