Material: Metal C-mount threads: 32TPI-UN-2A Built-in 0.5X reduction lens This 0.5X C-mount adpater is for microscope . It can be used in all eyepiece position with 23.2mm or 30mm or 30.5mm. Of course, it can be used in phototube and trinocular tube with 23.2mm or 30mm or 30.5mm.