PAD

Glossary Terms

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 by 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 overtop the shutter release on the front of a camera body. Finger pressure on its plunger closes down the lens diaphragm to the aperture selected on the f/stop ring, just before the shutter release is tripped.

With the 1959 appearance of the Nikon F and other Japanese cameras offering internally-coupled, instant-reopen diaphragms, the popularity of the PAD approach soon waned.