Why is everything blurry when I look through my rangefinder?
This is the number one issue for most new users, and the solution is almost always simple: diopter adjustment.
Think of the diopter as a focus ring, specifically calibrated to match your unique eyesight. If the image is blurry:
Locate the Diopter Ring: It's the knurled ring (often with a +/- scale) surrounding the eyepiece itself.
Calibrate It: Point your rangefinder...