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Old 12-14-2007, 02:23 PM   #1
nymoshigh
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Default What exactly does parallax free mean?

In reading a couple of threads related to optics I came across a phrase that I have seen a few times but never clearly understood. Would someone be kind enough to give me a little tutorial on what exactly a parallax free sight is? Could you also attempt to explain advantages / disadvantages of this type of sight also? Thanks for the input.
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Old 12-14-2007, 05:31 PM   #2
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Parallax is the apparent movement of the crosshairs in relation to your eye positioning. If you were to clamp a scope in your bench vise and line the crosshair up precisely with a bullseye painted on a distant object, if you move your head around very much, changing the angle you are viewing through the scope, the crosshairs would seem to move out of alignment with the bullseye. Parallax free means the optic is designed so this effect is minimized or eliminated, usually by setting it up so you can only get a full field of view when your eye is positioned in a very small "sweet spot" behind the lens.

Obviously, parallax can lead to sighting errors, so the less the better. An easy way to demonstrate the concept is to look out a window and focus on a distant object. Place a smudge mark on the window with your fingertip directly between your eye and the object (the crosshairs) and now move one step to either side and see what the smudge mark is lined up with. It won't be what you originally were looking at.
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Old 12-14-2007, 07:34 PM   #3
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Quote:
Originally Posted by KernelKrink
, usually by setting it up so you can only get a full field of view when your eye is positioned in a very small "sweet spot" behind the lens.
As quoted above, is this the principal behind the POSP type scopes? That your eye must be in the sweet spot to minimize the parallax? Is this what is so great about the Eotech sights? That you can move your head around and that you are on the same aiming point no matter where you move your head?

Is a cheek weld necessary for accurate aiming with a parallax free sight?

Are parallax free sights usually non-magnification?
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They must find it difficult........ Those who have taken authority as the truth rather than truth as the authority. -- G. Massey, Egyptologist

"Those who make peaceful revolution impossible will make violent revolution inevitable." -- John F. Kennedy

"One of the penalties for not taking an interest in politics is that you end up being governed by your inferiors". --Plato 400BC
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Old 12-14-2007, 08:47 PM   #4
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No experience with the POSP.

The Eotech and other "HUD" type sights project the red dot onto the lens and the lens itself is just there to act as a "projection screen" so you can see the dot. Imagine the sight as a laser sight beam being projected toward the target, no matter where you place your eye in relation to the target you still see the red dot on the target where the bullet will impact. All the lens is doing is intercepting that beam and letting you see the dot close up instead of on the target. Even if the dot is off center in the lens it is still lined up with the bullet path so cheek weld is not necessary with that type. Most sights of this type are non magnified because they are designed to be used with both eyes open, the brain automatically superimposes the dot over what is being seen by the shooter.

A normal scope has the crosshairs fixed, either etched into a glass lens or actual wires soldered into the tube. Moving your eye position changes the angle you look through the tube and thus changes the relative position of the crosshairs between your eye and the target. Scopes of this type can only be truly parallax free at a specific distance (some are adjustable), but in reality the error on a decent scope is so low within the normal ranges it is used for that it is inconsequential. This type usually has magnification.
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Old 12-16-2007, 10:11 AM   #5
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Thanks Kernel.
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"Those who make peaceful revolution impossible will make violent revolution inevitable." -- John F. Kennedy

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