I recently bought an ACOG compact 1.5x16 model, and I have a variety of conventional scopes in the low to middle quality level. I have noticed a big difference in how the ACOG works compared to the conventional riflescopes, and I want to know why. With a conventional riflescope, any model I've tried, the eye relief is a very narrow band - your eye must be at least 3 or 4" away from the eyepiece, and not much farther - the band is only 1/4 to 1/2" or so. Too close and the image shrinks to nothing rapidly and gets blurry. Too far and you're mostly looking at black space with a funny little image in the center, with blurry edges. This has been normal in my experience. With the ACOG, I am surprised to find that there is a much wider sweet spot of eye relief - I can move my eye within probably a 1.5" to 2" range and have basically the same sight picture. In addition, if my eye is too close I still have a perfectly good image that has not shrunk or become distorted. With my eye almost touching the lens I can see a square pattern of edges, which I believe is the edges of the prism inside the ACOG. If my eye is too far I do get the black circle around the edge of the image, but it doesn't get nearly as big, nearly as quickly as with a conventional scope. I realize that 1.5x is very little magnification, but I have compared the ACOG back to back with a 2x shotgun scope and the 2x shotgun scope behaves just like every other rifle scope I've used, in stark contrast to the ACOG. So, can anyone tell me what design feature makes the ACOG so much friendlier in eye relief? And are there any other scopes besides ACOG models that have this same friendly, wide range of eye relief with a good sight image?