T2 rings can be used to mount your DSLR (digital single-lens reflex) or SLR (mirrorless) camera on a telescope or microscope for primary focus photography.
It is important to have a T2 thread on the telescope side, this can be found on most focus strips, adapters etc. If not, you will need to purchase a suitable adapter (2"/T2, 1.25"/T2).
Also, you can use a T2 ring + adapter to capture images with your DSLR or SLR camera on your biological or stereo microscope, or you can use a T2 ring to attach such a camera to many spotting scopes optimized for nature observation.