Magnification vs. parallax

[Don357]

Now before I get started please don't flame me for owning an inexpensive optic. I know I should have invested more money.

A while back I bought a scope for my AR. I forget the name but it is a 2x6x28 compact (tactical) scope. When I was bore sighting it using a muzzle type laser, I noticed that when I sighted it in on the laser dot at a certain magnification and then changed magnification level, the point of aim would move rather drastically. I learned to compensate by zeroing at a certain magnification and just leaving it there, and it holds zero quite well. BTW the ocular focus had no effect on zero. I had read where the ocular focus could, on some optics, effect the focal plane adjustment which is why I mentioned this.
I would appreciate any constructive comments on this situation.

 

[powermad]

I noticed that when I sighted it in on the laser dot at a certain magnification and then changed magnification level, the point of aim would move rather drastically. I learned to compensate by zeroing at a certain magnification and just leaving it there

For Second Focal Plane scopes you zero on the highest magnification.

 

[powermad]

Kinda like this.
100 yd zero, 25 yds on 1 power.

25 yds on max power.

 

[JFrank]

I set eye relief at the highest magnification but zeroing could be accomplished at any power setting.

 

[Varminterror]

For any quality optics, the zero drift due to magnification change is nil. There is absolutely no reason to zero at any specific power for any optic which is free of mechanical defects - and frankly, that does include some pretty rot-gut, budget friendly optics (I have a couple $25-50 optics hanging around).

Parallax influence due to magnification, both with second and first focal plane scopes is imaginary - there is no optical or physical construct which would allow parallax error to worsen due to magnification in either reticle arrangement, and real world testing proves parallax error potential does not change due to magnification. The apparent “sliding” of the reticle only becomes more visibly recognizable at higher magnification, and the blurriness of the reticle (as not focused in the same plane as the target image) becomes more apparent, but neither are actually changing with magnification.

However, if you have any offset between the laser and the reticle center, as you zoom in and out, yes, in a second focal plane scope, the offset will appear to shrink and grow because your reticle is shrinking or growing relative to your target.

So if you keep your laser at the same point on the reticle, your reticle center will move farther from and closer to the laser dot on the wall as you zoom in and out. For example, say you put the laser on the target 2.5” below the center with your scope at 4x, dial the reticle to center target, and say the laser dot is aligned with the 25moa mark, when you zoom in to 8x, the dot has to move to the 50MOA mark with the reticle centered on target - but it will still be 2.5” below center on the target and the reticle will still be centered on the target.

So one of two things is going on:

1) User misunderstanding of how the laser will appear to behave in a second focal plane optic

or

2) Defective optic which is actually sliding around inside when changing magnification.

 

[Bill M.]

How far is the object the laser dot is printing? If it is less than 100 yards or so then your scope probably is not focused there and you will have parallax. You need to be in the distance range the objective is focused for. Ocular focus does not change that. A "better" scope will not change that.

 

[Varminterror]

How far is the object the laser dot is printing? If it is less than 100 yards or so then your scope probably is not focused there and you will have parallax. You need to be in the distance range the objective is focused for.

Expanding on this:

At 10 yards, in a scope with 100yrd fixed parallax and 28mm objective as described, the MAXIMUM parallax error is 1/2”. Every 5 yards is about 0.03” larger or smaller, 5yrs is .524”, 15yrds is .469”. So if you’re seeing an apparent shift of greater than 1/2”, it’s not parallax error. And frankly, it’s likely you wouldn’t even be able to unintentionally induce 1/2” of parallax error, as the scope’s FOV will warn you when you stray too far from center.

Maximum parallax error is accompanied by EXTREME shadowing in the scope, meaning it’s quite visible to the shooter, so achieving the maximum error on accident is pretty rare.

Here’s an example of 1,000yrd parallax setting being viewed at 10yrds. Pushing this around on purpose for this activity, I was able to slip the reticle around 1/2”-3/4” as estimated, with a calculated maximum parallax error of 0.71” at that distance - so I was able to achieve nearly the maximum possible parallax error for this scope, and the shadowing at the edges associated with such extreme misalignment of the eye from the center of the scope was impossible to overlook, even difficult to ignore.