Hi-Spy viewers only use manual focus. It is clearly the better choice.
And for a very straightforward reason. Auto-focus binoculars use one less lens in the internal optics and substitute it, in effect, with the lens in your own eye. And everyone’s eyes are different. If everyone had 20/20 vision that would be one thing but everyone does not. In tests here, we found three out of four users could not get as clear an image using an auto-focus binocular as they could with a manual-focus binocular. In fact, some could not get a clear image at all.
A manual focus, though more costly, allows each user to fine-tune the focus to their own vision.