Parallax at different zoom

[tangolima]

Do I need to recheck parallax when I change zoom? POI seems to change a bit at different zoom levels. Not much, 0.5MOA or so. Thanks.

-TL

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[LeverGunFan]

Is the parallax adjusted at the objective lens, or is it a side "focus" model? If it's an adjustable objective lens, I would think that the zoom would not affect the parallax setting. Not sure if the side focus can affect parallax.

[tangolima]

It is side parallax knob. First model I have. All AO before. Thanks.

-TL

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[tangolima]

Looks like it was my rifle. Its poi shifts as the barrel warms up, to the lower right by 0.2mil (0.7moa).

-TL

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[LeverGunFan]

Thanks for the update. Good to know it's not the scope, but not so good that the rifle is changing POI as the barrel warms up.

[RKG]

Strictly theory, but parallax in a rifle scope occurs when the focal planes of the target image and of the reticle are not the same. Increasing magnification might increase the apparent magnitude of parallax offset, but should not increase its actual angular movement.

[tangolima]

That's what I thought too. If the target image perfectly coincides with the reticle, zoom shouldn't matter. When I shot the string I did change the zoom, so that's why I thought it was parallax.

Cold bore shift is not uncommon. It happens with within 5 shots for rapid fire in my rifle. 0.2mil is a bit high. But as long as it is consistent, I can deal with that. It is interesting to see it happens. The first 2 shots are definitely cold bore to high left. 3rd and 4th shots ditter between high left and low right, seldom in between. 5th shot and on are consistent low right. If I wait 5 minutes between every 2 shots, the shot counts double.

-TL

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[jmr40]

Having POI change as magnification is changed is actually pretty common. It is more noticeable with some scopes more than others. Most of the time it isn't enough to matter in most hunting situations.

[tangolima]

Good point. Didn't think of that.

Previous trip to the range I changed the zoom. This time around I kept the zoom unchanged. That's how I realized it was the cold bore.

Actually I have started leaning towards fixed zoom scopes. Variable zoom doesn't really do much for me other than having a knob to monkey with. Everything seems simpler with fixed, say 10x, magnification.

-TL

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[Jim Watson]

Zero and parallax shifts used to be a common complaint with variable power scopes.
It was usual to adjust dead nuts at the highest power and figure a little shift mattered less at lower powers.

They have been improved but are not perfect, except maybe at the very top end.

[Rimfire5]

I find that increasing the magnification on a variable scope tends to make the focal point of the optical plane smaller than it would be at lower magnifications. Being off center on the optical plane would cause movement in your POI.

I realized, when I was teaching my grandkids to shoot, that I had to reduce the magnification when they first started to shoot because they couldn't find the image on higher power (24X).
As they go accustomed to the scope, I gradually raised the magnification. They had little trouble with finding the image once they were already in the right position relative to the scope.

Now many years later, I have found that on lower magnifications, you can barely tell you are in the center of the optical plane but being off the center will move the POI because your eye is at an angle to the reticle.
On high magnification, the optical plane focal plane is smaller,
but you can still set up slightly off center.

One thing you can do to determine if you are causing the problem is to lean your head back slightly until you see the black ring around the periphery of the scope image. If it is concentric and equal around the image, you are in or very close to the optical plane focal point. If the ring is not equally spaced, you should move your head until it becomes equally spaced and then move your head forward until the ring just disappears. If you get too close to the eyepiece, you will find that your POI will rise above your point of aim.

The second thing is to set up on your point of aim and close your eye for a few seconds. When you open your eye, check to see if your POA has moved. If it has, you probably were muscling the rifle a bit. Move your body not your head until you do not see any difference before and after you close your eye.

Either case, being off center of the optical plane or aiming at a point that is not your true aim point, will move your POI. It may not be scope parallax, just being off center on your scope's optical plane.

[tangolima]

Maintaining center of the eye box is the go-to method to avoid parallax problem with scope with fixed parallax. For scope with parallax knob, it becomes less of a necessity, even though it is a good habit to keep. Magnification may not make thing worse as it doesn't magnify angle. That's how I understand it.

I agree too much magnification doesn't help. It makes the sight picture wobble so much that it becomes uncorrectable. I always try to use minimal zoom. But at time the temptation is simply too strong to resist.

-TL

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[Scorch]

POI shift is not uncommon in variable power scopes. As the lenses move to increase magnification the POI changes because the lenses' center point are not perfectly centered and aligned with each other. Pick a magnification and sight it in for that magnification and then know how much and which direction POI shifts as the magnification changes.