Scope and the front site post - Why can't I see it?


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Wade-0
May 3, 2008, 12:56 AM
So I put a new scope on my AR-15 today, and was really surprised to find that the front site post, which is sticking up half way in front of the objective, is completely absent from the sight picture. I've been searching around for a basic scientific explanation of how this works, but I haven't been able to find anything. Does anyone have any links to good resources, or a good explanation of what makes this giant black steel obstruction disappear at magnification over 3x?

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Dstoerm
May 3, 2008, 01:35 AM
Depth of field. The front post is too far from the focal point of the optics to be visible due to a lack of depth of field - both your eye and the scope are focused way past the end of your barrel. If you could in fact see the post, everything past the post (ie- your target!) would be horribly out of focus.

In photography, you can indeed shoot a picture through a tight chain link fence, when the lens is focused on a distant subject, the fence disappears. Try putting something wider across the post, and it will be an visible out-of-focus blur.

This of course assumes that the post is truly in the line of sight of the scope. Many times the scope is mounted above the post.

Kind of Blued
May 3, 2008, 03:35 AM
Granted, there is a portion of the sight picture that is not visible, regardless of where you are focused.

General Geoff
May 3, 2008, 03:45 AM
Hold the rifle way out in front of you, and look through the scope. With a few feet of eye relief, you'll be able to see the front post pretty easily in the reticle.

RoostRider
May 3, 2008, 05:13 AM
Magic?

Somehow I knew this would be an AR question.... I wondered the same thing when I put a scope on my flat top....

Wade-0
May 3, 2008, 08:27 PM
It does seem a little like magic. :o Even with the explanations, it's still hard to believe.

Fly320s
May 3, 2008, 10:22 PM
Not so magic.

Do you ever wear glasses of any type? Have you ever worn dirty or scratched glasses, but didn't notice the dirt or scratch while looking at something in the distance?

Same idea.

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