Female connection tube for BALPRO-adapters. Allows the connection of two extension tubes PRO50 or other BALPRO connection adapters. The combination of PRORING, 2 tubes PRO50, the PRO-APO-DIGI 90 and the matching camera connection adapter gives a fixed magnification of 1:1. Thanks to the modularity of the system, even longer and incredibly robust macrotubes can be mounted
