I have a Leupold 12-40 GR with the PhoneSkope, it works, but not as good as PhoneSkopes do on most other spotters. The adapter needs to slide further onto the focus ring, it doesn't grab solid enough for horizontal orientation of the phone. The weight of the phone rotates the adapter down to a vertical orientation. The fact that the adapter is on the focus ring of the spotter also means it has to change position whenever you adjust focus. I contacted PhoneSkope about the adapter ring needing to be deeper and grabbing more of the spotter, but it never got anywhere. The GR 12-40 is a good spotter; the PhoneSkope is a great product - the two together unfortunately aren't the best pair, but still a good pair.