This black ring is referred to as vignetting and is to some extent inevitable when digiscoping. There are, however, a number of things that can be done to minimize it:If you are using a phone, you can eliminate vignetting by simply zooming in with the phone’s camera.Check the alignment of the adapter to make sure your phone or camera is looking straight into the eyepiece of your spotting-scope or binoculars.Make sure your adapter is mounted as far forward on the eyepiece as possible, placing the camera as close to the eyepiece as possible. Some eyepieces have rubber eye-cups than can be removed to allow a better fit.