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Old February 20, 2013, 05:39 PM   #8
Bart B.
Senior Member
 
Join Date: February 15, 2009
Posts: 4,682
timelinex, your explanations' one of the best I've seen. You know more about rifle scope optics than most.

But I made no assumptions. The image is parallax free when the scope's focused on the target. That puts the target image plane on the scope's reticule. It mattes not about optical abberations as that only defines the clarity of the image; how detailed the focus it or the focal lengths of all the individual lenses. And the scope's eyepiece can be removed and no human eye behind it and the objective lens stuff still focuses the target on the reticule. Focusing the scope on the target only involves those parts of the scope from its reticule forward. The scope's eyepiece makes up for the human eye not being able to focus on the reticule that's only 4 to 5 inches away from it; it's just a magnifying glass.

All the eyepiece does is compensate for the aiming eye's vision regarding its focal length. It's job is to correct the aiming eye's optical properties so the reticule is sharp; that's focusing the reticule on the eye's retina. It matters not what vision numbers the aiming eye has; 20/20, 10/20 or 40/20 without correction. The eyepiece will correct for that.

It's best that when someone gets a rifle scope, the first thing they do is look through it at the clear blue sky then focus the eyepiece so the reticule's very sharp. If the reticule ain't sharp the instant one looks into the scope aimed at a clear blue sky, adjust the eyepiece until it does.

My original comments related to the front to back movement of scope lenses focusing the target image on the reticule. Parallax is a situation one one of the optical devices in the aiming system is off the scope's optical axis at right angles to it. That's the human eye not aligned with that axis and the scope's not focused on the target focusing it on the reticule. Nothing in the scope moves anything at right angles to the optical axis. E and W adjustments move the optical axis in the scope for getting zeros and do nothing focus wise.

Rifle scopes focusing for target distance are exactly like camera lenses focusing for subject distance. One puts the focused image on a reticule and the other puts it on a sensor (chemical or photocell). Rifle scope's eyepiece does the same thing as a SLR camera's eye piece; focuses the target/subject image on the human eye's retina correcting for the human eye len's focus characteristics.

RGPM1A, when there's no movement of the target realtive to the reticule after you've focused the scope but it's not sharp, that's because the scope'e eyepiece isn't set right for your vision; corrected or otherwise. Take your scope outside and look through it at the clear blue sky then focus the eyepiece so the reticule's very sharp. If the reticule ain't sharp the instant you look into the scope aimed at a clear blue sky, adjust the eyepiece until it does. Then put it back on the rifle and focus parallax free on a 100 yard target. The target image should be a sharp, well defined image.
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Last edited by Bart B.; February 20, 2013 at 06:40 PM.
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