The BR-2A enables you to mount a 52mm front thread lens in reverse on the PB-6 and Nikon cameras. This increases magnification and working distance. BR-3 provides a 52mm thread for filters and other lens attachments with lens in reverse. BR-5 allows mounting lens with 62mm thread (use with BR-2A). BR-6 for the PB-4, PB-5 and PB-6 enables Nikkor lens auto-diaphragm control with BR-2A BR-3 AR-4/7/10 cable release.