For video applications, you need a 75 Ohm plug.
Almost all other
appications (test equipment, ethernet) use 50 Ohm.
Sounds a little off. Cables are important, with impedance
varying with diameter, _Terminators_ are important, coming
in the 50, 75 Ohm varieties mentioned, but the connector
itself being 50 or 75 Ohm? First I've heard of this.
Every catalogue I've looked in lists 50 Ohm and 75 Ohm BNC connectors. It
may just be that they're designed for a partticular type of cable, but
the chassis mount sockets _also_ esist in both versions, so presumably
there are differences. It may just be in the outside dimensions of the
inner contact so aas not to give a tiny mismatch at that point.
-tony