Which adapter do I choose, Universal or Custom C-3?

It depends on what you are trying to do with your smartphone. If you are planning on using your phone with one spotting scope, pair of binoculars, or other optic, our C-3 Custom Adapter is right for you. If you plan to use a variety of optics with different sizes of eyepieces, the new U1 and U2 Universal Adapters will work better for you than a C-3 Custom Adapter.