I ripped this off another forum:
What you are experiencing is a common thing called Vignetting. I have an I phone XS Max. You have to use your zoom to adjust for it. As you zoom, it will fill the view finder. If you zoom too much it will do the same thing. You want about a 3.5 zoom. It will vary a little. Here is a little copy and paste section from the net that gives some explanation and how to overcome it:
Selecting a digital camera for digiscoping is different than picking one for general use. Excluding the professional digital SLR cameras, the best cameras for general use have large objective lenses with large apertures. For digiscoping cameras, however, large objective lenses lead to one of the biggest problems in digiscoping - vignetting.
Vignetting is the effect caused when the entire frame of the image is not illuminated, leaving a circular image with surrounding black. It occurs when either the objective lens of the camera is larger than the exit pupil of the scope, or when the curvature or mounting of the objective lens of the camera is farther away from the eyepiece of the scope than the scope's eye relief. For this reason, most digiscopers prefer cameras with small objectives, which can be brought very close to the scope's eyepiece. In general, pocket-sized digital cameras with large zoom ranges are also problematic, as their zooming lens elements recede from the eyepiece when zooming to larger magnifications.
The only way to solve the vignetting problem without changing the physical parameters of the camera and scope is to crop the image. Cropping can be done in a photo-editing program, which reduces the size of the image, or by zooming in with the camera while taking the photo. Zooming in maintains full resolution of the image file, but lowers image brightness.