Changing magnification on scope and POI

T

Twohawks11

New member
Newbie here....
If I change the magnification on my scope while shooting a 100 yard target does the POI change too?
Ex. Shoot at target from 100 yds at 4x magnification and then 100 at 9x mag would the Zero be the same?
 
In the past, it was more of a problem than currently(especially with cheaper grade scopes). Modern machining and production techniques provide lower cost scopes with nearly the same potential as older, higher cost scopes.
FWIW
I always sight-in at the highest magnification on the scope and usually hunt at the lower end. I haven't noticed a significant problem with even moderate priced scopes in the last 20 years or so.
 
If anything your groupings should tighten on the scopes dial up. Been awhile since I took notice of my rifles POI. As I recall it changes some maybe 1/4" or so from 5 to 10 power at 100. Another issue. Wind. Many times over looked but still a sneaky trouble maker for shooters.
 
I do slightly different from Mobuck. I set my scope at the middle of the range for sighting in. It may not gain me anything but I figure that this way I've got the least amount of room for possible change in either direction.
 
Most variables have a small amount of reticule shift when magnification is changed. That's caused by the two lens groups at the back end moved back and forth in a tube by the power ring's two camming mechanics having a tiny amount of tolerance in their position. Good scopes will have less than 1/4 MOA error. Others may be one MOA or more.

With an optical collimator in the muzzle or the rifle clamped solidly, focus on something then turn the zoom ring from stop to stop a few times. Watch the reticule make figure 8 loops about the aiming point.

Recoil can also change those two lens groups positions and the barrel won't point to where it did for the previous shot with the reticule on the same aiming point. Errors are the same amount tha changing power causes.

The errors happen when the lens groups are off dead center or are crooked in the tube they slide in. This makes the target image move at right angles to the optical axis on the reticule. It has nothing to do with what causes parallax; the objective lens group at the front being set wrong for target range causes parallax.
 
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The only way to know for sure if your scope shifts point of impact is to try it. Shoot a 5 shot group on the lowest magnification, then shoot another 5 shot group on the highest. I have only had one scope that had more than MOA difference, and it was a very cheap one.
 
Allen, what if you can shoot 1 MOA groups on average, they're not all the same size and their centers vary about a 1/2 MOA area? How precise does that measure the scopes repeatability?
 
I'll present a idea for ridicule. Insert a laser bore sight into the end of the barrel and change the magnification while looking for shifts in the POA?
 
Some people have told me, and I also experienced it, that changing the focus piece changes bullet impact. I suspect it is the change of "Stock to cheek weld" as you adjust your eyes. Could this be what is happening when you bump the scope power up and down?
 
To be precise, changing the power on a scope will NOT change your POI (point of impact).

The bullet is still going to go in the same place it did before.

What may / does change is you precise point of aim (POA), in relationship to the POI.

If you want to see how your personal rifle and scope perform, simply shoot a group at each of the power setting you use, while keeping the same point of aim for all shots.

See where the groups are at 3x, 6x, 9x (for example) using the same aiming point (and of course the same ammo).

It might surprise you.
 
Im pretty much where Mobuck is and I also practice what 44AMP says to try...,
I dont notice anything different with my optics.
 
I have several budget scopes ($150price range) I have Redfields, Nikon's, and a Vortex. Recently I tried some of them to see if I had a POI shift as magnification was increased and decreased. At 100yds on 9x I was hitting bulls eyes. I moved to fifty yards on 9x and hit exactly where it should. I moved to 3x and it hit in the same place.

Short Answer: I haven't noticed any.
 
I have put several variables on a collimator and they all move the target image about the reticule when changing power. The scope tube and rifle don't change their relative position to each other. But the aiming axis moves around. If that means point of impact won't move around the aiming point, then someone has to rewrite the laws of optical physics.
 
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It happens, but most of the time the difference is so small than most rifles, and shooters aren't good enough for it to be a factor. Especially if you are talking about the level of accuracy needed for a hunting rifle fired in field positions.

To help eliminate as much possibility as I can I always zero my scope on the highest magnification. If I need to take a longish shot that is where my scope will be adjusted anyway. I then practice with the scope set at the lowest and highest settings just to know. I NEVER use any of the settings between highest and lowest.

On shorter range shots, out to 100-150 yards I do just fine on 2X or 3X and minor differences at those ranges aren't enough to cause a miss. On anything longer I go straight to 7X, 8X, or 9X depending on the scope where it was zeroed. While the differences at the mid range settings are likely to be small, or not at all, It is just one less variable to worry about.
 
"Insert a laser bore sight into the end of the barrel and change the magnification while looking for shifts in the POA"

A good idea BUT most laser bore sighters have neither the range or precision (size of dot) to see such a small change. My bore sighter (albeit a cheap piece) has a range of less than 50 yards and a dot about 2" diameter at that range. There may be others which do a better job but still not close enough for a .25-.50 MOA change.

"To help eliminate as much possibility as I can I always zero my scope on the highest magnification. If I need to take a longish shot that is where my scope will be adjusted anyway"

Kind of what I'm thinking, also.
 
An easy way to see how much the reticule figure 8's about the aming point when power's changed is to put a monocular (binoculars, telescope) on the scope's eyepiece. Focus it on the reticule and with 7X or more magnification, the tiny figure 8 movement of the reticule's easier to see.

This is also a way to see small parallax errors as you move its look angle on the eyepiece.

Yes, the monocular will focus on the reticle that's about 2 inches in front of the scope eyepiece lens. That lens makes the reticle appear to be a long ways away. Light from it to the aiming eye is the same a that from a distant object.

One other issue with those two metal tubes camming their lenses back and forth in the erector/zoom tube changing magnification. The shock of recoil repositions them a tiny amount, too. I've seen that on my scope collimator I made when I smacked its front with a mallet sliding the 3-foot long base duplicating rifle recoil force.

Pretty good video on variable scope erector/zoom tubes:

https://m.youtube.com/watch?v=npL1sInLdxM#

It also shows the tube's front end against the adjustment turret screws. They're spring loaded so they get jostled around from recoil. It helps to have them reset to the exact same place for every shot. Which means any error in positioning the tube's front end adds to the reticule's repeatability from shot to shot. It's a much smaller amount than zoom lens position errors.
 
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