Eye box describes the ability to easily see both a clear outline of the outer edge of the scope (you should see a uniform "shadow" around the entire circumference) and a clear reticle at the same time. Problem with this one was that the eyebox had a very critical, limited "field of view" so that upon recoil (and he was using a can on a 260, so not much recoil) you lost focus through the scope and it was difficult to regain it in time to see where the shot hit the dirt. Devin his first step is to send it back to Vortex to get checked out, but that is a major disappointment for a scope > $2000. I have a Viper that is ok, but I would say is also more critical in this regard than my other scopes. Hope he gets it fixed; Vortex has a great customer service reputation.