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Posted: 9/16/2004 7:48:46 PM EST
I have an aimpoint that is supposed to be 'parallax free'. I have an OTAL, which IS parallax free.

Why, when both are on and I move my head away from the centerline of the rifle, does the aimpoint dot move farther away from the laser dot? It does this whether I move up, down, left or right.

Anybody?
Link Posted: 9/16/2004 10:21:34 PM EST
[Last Edit: 9/16/2004 10:34:33 PM EST by zrxc77]
In my understanding the Aimpoint is not truly 100% parallax free despite claims to the contrary.

Aimpoint says that most red dot sights will exhibit angular deviation between the optical and mechanical axes of the sight when you shift your eye position away from the center of the sight:



Note that because of this angular deviation, the greater the distance to your target, the greater the shift in POI from POA.

Aimpoint's design, conversely, keeps the optical and mechanical axes parallel even when you shift your eye position:



Presumably this is why Aimpoint claims it to be parallax free. Note that despite the fact that the optical axis remains parallel to the mechanical axis, the POI still shifts by the amount you move your eye. The shift isn't much (+/- 15 mm from center for a 30 mm diameter tube), but it is there. However, in contrast to the design shown in the first diagram, the Aimpoint's shift in POA/POI does not get worse as distance to target increases because the optical and mechanical axes are parallel.

That is why people will say that the Aimpoint is effectively parallax free beyond 25-50 yards or so. At greater distances the offset induced by moving your eye becomes negligible when measured in angular MOA. For example, a 15 mm offset at 50 yards is roughly 1 MOA, already small compared to the 4 MOA diameter of the Aimpoint dot. Furthermore, an absolute offset of 15 mm maximum at any distance is truly negligible for the intended use of Aimpoints - combat and hunting of medium to large game.

Try comparing your Aimpoint to your laser on a target that is further away and see if the parallax error becomes negligible.

To read more about Aimpoint's design, go here


<edited to replace original background color of Aimpoint's graphics with an easier to see color>
Link Posted: 9/16/2004 11:18:03 PM EST
That was good info.

Do you have similar info for the EOtech, which is what I have?
Link Posted: 9/16/2004 11:41:14 PM EST
[Last Edit: 9/16/2004 11:42:20 PM EST by eodinert]
Actually, between the time that I posted this and the time that I read the response, something occured to me. I had heard at some point that the 'focal plane' is forward of the rifle, and there should be less error at longer distance. Zapping the neighborhood trees in the distance seemed to confirm this.

Or, what you said.

My faith is restored.

~doug
Link Posted: 9/17/2004 4:49:50 AM EST

Originally Posted By JeffersonS:

Do you have similar info for the EOtech, which is what I have?



My understanding is that an optic can be designed to be completely parallax free at a fixed distance, and an optic can be designed to have minimal parallax error at all distances, but no optic can be truly, 100% parallax free at all distances.

Now, the EOTech uses holography to project its reticle so it isn't exactly the same as an Aimpoint. However, I believe the end result is the same. Both optics have to project the reticle/dot to appear as if it originates at some distance, and I think both pick infinity (or at least a long range) as that distance. Thus, they are parallax free at that long distance and suffer some degree of parallax error at shorter distances. However, their designs minimize that error in absolute POA/POI shifts to the point where it is negligible for practical (read "combat") applications.

Try an experiment yourself. Sight your EOTech on a point 50 yards or more away and then on a point a foot or two away. In both cases shift your eye position without moving the EOTech and see if the apparent POA shifts with your eye. I suspect you will find, as eodiner did, that there is no appreciable shift at the long distance and minimal but perceptible shift at the shorter distance.



Originally Posted By JeffersonS:

Actually, between the time that I posted this and the time that I read the response, something occured to me. I had heard at some point that the 'focal plane' is forward of the rifle, and there should be less error at longer distance. Zapping the neighborhood trees in the distance seemed to confirm this.

Or, what you said.

My faith is restored.



Rats. So I guess I can't offer to take that defective Aimpoint off your hands for cheap.
Link Posted: 9/17/2004 12:42:12 PM EST
Good info, thanks, zrxc77.

On a slightly related note, one of the things I have found interesting is that the dot from the OTAL and the dot from the aimpoint look nearly identical through the aimpoint, if the aimpoint retical is adjusted to a brightness to match the laser. It looks like the aimpoint has two dots. Or, an OTAL is like having an aimpoint dot all the time, even when your head is not lined up on your sight... at least when the sun is not out. Still need the aimpoint for bright sunny days.

If anybody out there is considering an OTAL, go for it. It's a rockin' piece of kit.
Link Posted: 9/17/2004 6:07:52 PM EST

Originally Posted By eodinert:

Good info, thanks, zrxc77.


You're welcome.



Originally Posted By eodinert:

... an OTAL is like having an aimpoint dot all the time, even when your head is not lined up on your sight...


There are probably many who would say that an Aimpoint is like having a laser sight on all the time, except that there is no (or very minimal) target side signature.
Link Posted: 9/17/2004 6:41:18 PM EST
good post zrxc77...

aimpoints are 'good enough' at being paralax free
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