Paralax: what is it?

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powderific

New member
What is paralax?

I'm sure that this question has been asked before, but I'm feeling lazy :p All I've been able figure out so far is that it has to do with the point of impact changing with a scope. How does it change? Why does it change? Are there little elves in my scope that move the crosshairs around? If so, why are some red-dot sites paralax free? Does the red dot scare away the elves? Or is it just more difficult for elves to hide in red dot scopes?

Thanks in advance!
 
Elves.... no...

Parallax is the difference in view between two points looking at an object.

Ever drive down the road by two rows of trees? The closer row seems to move quickly, while the distant row seems to move slower. This is parallax.

Look at something a few feet away from you. Close one eye. Now switch eyes. See how the "view" changes? This is parallax.

Parallax is good for astronomers - it lets us measure the distance to stars by measuring the shift in view when the Earth is on opposite sides of the sun (Heliocentric Parallax)..

OK - too much information...

Anyway, parallax, as it affects rifles, is the difference in the "view" of the scope as compared with the "view" of the rifle bore.

That difference, while seemingly insignificant, makes all the difference in the world when you're talking about 1" at 500 yards!

Hope that helps (and doesn't confuse you more). I was an astronomer before I was a shooter. :)

-LevelHead-
 
OOooh! ooH! i get it now! Sortof like rangefinder cameras. (or any rangefinder, i spose) Thanks LevelHead! Until now I hadn't ever heard a description that made any sense.
 
Great so far. But then we have parallax error that is often found within the optical system itself. Even with single eye structures like telescopes and camara lenses. Results the same, distortion of the perceived image.

Sam....help, I opened the can but can't cook it.
 
Take a cheap scope, pull your eye back away from it a little, and then move your head from one side to the other or up and down a little. The point of aim changes within the scope (crosshairs not on target anymore) even though you didn't move the weapon.

That is parallax.

Get a top of the line scope and it is drastically reduced if not totally eliminated
 
Parallax is the difference in the focus plane of the target and the recticle. In a scope corrected for parallax at a given range, the recticle is on the same plane (virtual) as the target. No matter where your eye is in relation to the center of the optical axis, the impact point equals the aiming point.

When there is a parallax error, the only time the impact point is the aiming point is when your eye is perfectly centered on the optical axis of the scope. Think of parallax error like your car's speedometer needle in relation to the scale. If the needle shaft is considerably long so that the needle is a good distance from the scale, the only time you can accurately judge spped is when your eye is directly over the needle. Move your head and the indicated speed changes even though the car's velocity is constant.

Adjustable objective scopes (scopes with an adjustment on the bigger front lens) allow one to correct, bringing the recticle on the same focal plane as the target, optically melding the recticle to the target. Red dot scopes have great hyperfocal (big word meaning deep focal plane) distance due to low magnification and corresponding smaller objectives.

Parallax is most critical on high magnification, large objective scopes and short-range work like air rifle scopes.

You can observe parallax if you have a rifle cradle. Center the recticle on a target and then move your head to shift your eye off the optical axis. You will see the aiming point move even though the rifle has not.
 
Great explanation, Keith. A couple of rifles that I have are equipped with Leupold AO scopes, which have parallax setting for various distances. What's odd is that, when I shoot at 100 yards, I have to adjust the objective for parallax to a setting other than the 100 yard mark on the tube. Usually somewhere around the 75 yard mark. And our 100 yard backstop has been measured carefully.

Any ideas why this would occur? Leupold hasn't been able to give me an answer other than to send the scopes back.
 
Oh yes...

The dreaded scale problem. The focus mechanism is out of whack. I had a scope that I thoughtfully openeed up to clean. I managed to get it all back together (clean and with a fresh N2 purge in a glovebox). I measured a 30 yard range with my tape, set the scope on a tripod and adjusted the parallax out. I reset the scale(small screws on the scale barrel) to 30 and though all was well. Get to the 300 yard range and I have to set the scope at an indicated 200 to get the parallax out. No biggie, just a "fine tune". Then I go to 100 yards and I have to set the scale to 300? So I adjust it back to indicate 100 and now the thing stops before I can indicate an infinite distance. So I then rack the booger all the way to the infinity stop, set the fool thing up at night pointing at a star in the clear sky and rack the power to max. That fixed it but I had to break seal and no more N2 purge. Drat, another trip to the lab.

Moral here? Don't open scopes. I'll stick to fixed power camera lens repair/cleaning.
 
Keith, you're gutsier than I am. No way I'd open up a scope.

What's weird about this is that both scopes--a 6.5x20x40mm and a 6.5x20x50mm--have to be adjusted to the same setting, approximately 75 yards.

The idea of fiddling with the adjustment at anything other than known ranges seems a bit dumb to me. I mean, if ol' Osama just peeked around the 75 yard backstop, how would I have time to fiddle with the parallax adjustments?
 
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