AR15 Front Sight ...In front of Scope

Discussion in 'Long Range Scopes and Other Optics' started by TnTom, May 4, 2011.

  1. TnTom

    TnTom Well-Known Member

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    Will someone explain why the front sight on an AR does not block the view through the scope when the scope is mounted in the same line of sight as the front sight?

    Seems to me it would mess up the view but I cant see any impact. Weird.
     
  2. brentc

    brentc Well-Known Member

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    Basically, it's because the increased magnification of the scope focuses your eye to objects further away allowing the field of view to "bend around" the front sight. If you have a scope with low enough magnification you will still see the front sight post when looking through the scope.
     

  3. joseph

    joseph Well-Known Member

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    Light & time bends. "Movement through time is possible. Einstein proved it. In theory." Martin Sheen in the movie "The Final Countdown". :D

    joseph
     
  4. TnTom

    TnTom Well-Known Member

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    well that is enlightening!lightbulb
     
  5. LouBoyd

    LouBoyd Well-Known Member

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    Actually the front sight does block some light reaching the eye. The amount of blockage is the area of the front sight subtracted from the area of the objective lens. Probably a loss of around 20% of the light. . Since the human eye auto-adjusts for brightness it's not noticable at all. If the front sight were shiny it could scatter light into the field of view, but being black that is unlikely to be enough light to be noticable.

    There is a bit of bending of light caused by the front sight. Diffraction effects will be present but usually too small to notice. In photos of stars you'll sometimes see "differaction spikes". Here's an extreme example:
    [​IMG]

    In this photo of the star Sirius the four spikes are from light diffracted from the support vanes which hold the telescope's secondary mirror. There are in an equivalent positon to the front sight in front of a riflescope. It is not an image of the vanes themselves as they are completly out of the focal plane. If you looked at a distant bright point of light with the riflescope you could see simialar but weaker spikes s caused by the presence of the front sight. It would be a single thin white horizontal line with a little haze around it.
     
    Last edited: May 10, 2011
  6. TnTom

    TnTom Well-Known Member

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    That's very interesting and I appreciate the explanation. I will see what happens looking at a distant yard light, its about 200yd away just for fun and see what I notice or must it be at a much greater distance. Thanks again. Tom