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Old October 25, 2013, 04:08 PM   #1
FLChinook
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Should changing scope power change POI?

There may be no easy answer to this... it may depend on the quality of the scope... but..

Should changing the power on a variable power scope change the point of impact? Does it depend on whether the change takes place in front or behind the reticle?

I've changed the power on my Swarovski 3-12x50 without any concerns about POI but I just had an experience with my Zeiss Conquest 4.5-14x50 that suggests otherwise. Going from 14x to 6x to 8x all seemed to change the POI. Is this possible? Do you have to sight in a variable power scope at the power you expect to hunt with and never change it?

I also noticed a change in POI when I changed the variable parallax knob from 100-yd to 200-yd but I didn't do enough shooting to confirm this...

I was shooting a .257 Weatherby Mag from a bench rest; this gun regularly shoots under 1 moa.
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Old October 25, 2013, 04:47 PM   #2
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The short answer is no, it shouldn't. However, on some scopes it does. The manufacturer can make the scope so that there isn't a noticeable movement of POI. If I owned a higher end scope (I own several) and it was doing this, I'd be sending it back to the manufacturer. I had one scope do this about twenty years ago and sent it back. Got it back corrected.
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Old October 25, 2013, 04:47 PM   #3
AllenJ
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Quote:
Should changing the power on a variable power scope change the point of impact?
No, but sometimes I've seen it also. It has always been with cheaper scopes though and I'm very surprised you're seeing with a Zeiss. How much change are we talking about? And, 257 Weatherby is a barrel warmer, any chance this POI change was due to barrel heat creating mirage?

Quote:
Do you have to sight in a variable power scope at the power you expect to hunt with and never change it?
You should not and if the Zeiss is truly doing what you think I'd call them to discuss an exchange.
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Old October 25, 2013, 05:13 PM   #4
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I'll have to do more testing. I only went to the range today for a final tune-up before hunting season and I did not have my usual compliment of ammo.

I was, in fact, ready to leave (quite happy) when I decided to fire one round at 6x (after finishing a very nice group of about 1/2" at 14x) and that shot was at 10 o'clock about 1.5 " away from the main group. Almost out of ammo, I tried one shot with the scope at 8x and it came in at 6 o'clock about 1" away (2.5" from the 6x shot).

Of course, one or two shots prove nothing and these variances are clearly within the range of prior experience. But for safety sake, I'm leaving the scope at 8x for opening day and I'll bring more ammo to the range next time to fully test out the issue.

If only I hadn't decided to fire those last two shots...
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Old October 25, 2013, 10:52 PM   #5
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If you can lock or hold the rifle in a rest so it won't move you can turn the power adjustment while looking at the reticle. Focus on something at 100 yards and see if the reticle moves around. If it does contact Ziess. If not shoot it some more, let it cool, and shoot it hot. See what happens.
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Old October 25, 2013, 11:04 PM   #6
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Should changing scope power change POI?

I have a Zeiss Conquest 3-9x and think I've seen this phenomenon occur as well. I was wondering the exact same thing you were. The rifle is a rem 700 and in a really flimsy factory stock . I want to get it into something like a McMillan or B&C or similar before making any final assumptions about POI moving and bothering Zeiss.
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Old October 25, 2013, 11:16 PM   #7
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Changing powers will slightly shift POI on virtually every scope ever made. In order to really see it you'd have to have an optical collimator.

As you zoom the scope in and out, you'd generally see the crosshair move in a figure-8 pattern.
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Old October 26, 2013, 12:30 AM   #8
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Technically, the POI does not change. What is changing is the POA.
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Old October 26, 2013, 08:57 AM   #9
FLChinook
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What is POA? Thanks
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Old October 26, 2013, 09:27 AM   #10
Brian Pfleuger
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Point of Aim
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Old October 27, 2013, 07:28 PM   #11
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While technically yes, the amount of change should be so small as not to matter. Especially at close range. I never use any of the power settings on any scope other than the highest and lowest. I zero at the highest setting. I'll be using it for longish shots anyway and I don't have to worry about POI changing because i will be using the same magnification I zeroed it with.

I keep the scope set at the lowest setting almost all of the time. Even if I do get 1/4-1/2 MOA difference it simply isn't an issue at close ranges. I never fool with anything in between. No chance for POI differences and no reason to introduce more variables. If it is too far to use 2x or 3X, and if I have time to adjust the scope then I might as well go to the maximum magnification where it was zeroed.
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Old October 27, 2013, 08:10 PM   #12
Brian Pfleuger
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That's essentially what I do.
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Old October 28, 2013, 02:28 AM   #13
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Something that could be a factor:If the rings are not truly co-axial,there will be some tube distortion when the rings are tightened.

The scope may have been made very precisely,tolerances in the .0002 in range,but if you distort the tube .002 in,you may introduce variation in POI vs POA
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Old October 28, 2013, 06:10 AM   #14
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Your very first opening sentence answered your question. Only on a cheap scope will that happen. I have a cheapo on my 223,that it will happen to. I have a Sightron on my BR. It does not happen to.
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Old October 28, 2013, 09:23 AM   #15
Double Naught Spy
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Quote:
Your very first opening sentence answered your question. Only on a cheap scope will that happen.
Um, no. I have definitely seen this with a $1500 Nightforce scope at my range. We even tried it on a different rifle with different rings and with different shooters. Full zoom at 100 yards was more than 3 MOA off than least zoom at 100 yards.
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Old October 28, 2013, 01:39 PM   #16
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Your very first opening sentence answered your question. Only on a cheap scope will that happen.
Um, no. I have definitely seen this with a $1500 Nightforce scope at my range. We even tried it on a different rifle with different rings and with different shooters. Full zoom at 100 yards was more than 3 MOA off than least zoom at 100 yards.

Ok. I was looking at getting a Nightforce too. Maybe just another Sightron will be in the bag instead
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Old November 6, 2013, 12:06 PM   #17
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I've checked a bunch of variables for reticule movement zooming power back and fourth. All price ranges of scopes had some. A couple of expensive ones were the worst.

Testing variables for reticule shift from recoil showed all variables to have some, but it was least with power set hard into one limit. Fixed power scopes had the least, if any as most had none.
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Old November 6, 2013, 04:28 PM   #18
FLChinook
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Quote:
I've checked a bunch of variables for reticule movement zooming power back and fourth. All price ranges of scopes had some. A couple of expensive ones were the worst.
This is very disturbing news It almost negates the value in buying a variable power scope.

Do you know if there's a source for detailed numbers on this?

Are you willing to share some of the brands/models that you have experience in showing this affect?

As I said in my first post, I am using two main scopes now, a Zeiss Conquest 4.5-14x50 and a Swarovski 3-12x50. The Zeiss I have limited personal data for. The Swarovski is on my .300 Win Mag and I don't generally shoot 100's of rounds through this gun. I'm going to email Swarovski and ask them what their position on this is.

The only other scope I can "bring immediately into play" is an old Leupold M8 3x (made in USA). I had this scope on a beat-up .338 Win Mag and someone offered me more than the gun was worth because he just wanted the scope. I told him I wanted to keep the scope and he lost all interest in the gun... :roll eyes: The trouble with this scope is my old eyes can barely make it work to 100 yards. But I bet if I could get by with the low power, it would be the ultimate in POA reliability...

Thanks
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Old November 6, 2013, 08:04 PM   #19
Bart B.
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FL, it was some years ago that I checked 2 Leupolds, 1 Redding, 2 Nightforces, two Weavers and 3 or 4 others for hysterises loops (figure 8 movement) of scope reticules when changing power on my bench collimator I built. I've since checked a few others using an optical collimator in the muzzle. Note these were all second (rear) focal plane reticule focus.

The best of those earlier ones was the Weaver V14, a 4-14x scope that had only about 1/3 MOA error. One Leupold had about 1/2 MOA and the other near 3/4 MOA error. Both Nightforce scopes had over 1 MOA error as did the Redfield 8-32X prototype target scope they had given to an Olympic Team member to check out.

One other thing I checked was range focus accuracy on the reticule indicated by zero parallax. Locking the scopes on a counter top aimed at things outside 100, 200 and about 10,000 yards (for the infinity setting) away, I would set the scopes objective lens barrel to those ranges then with a magnifier, look through the eyepiece for parallax as I moved my aiming eye around. The Leupolds were pretty close to perfect at the shorter ranges but a bit off at infinity and the Weavers were close seconds. The Redfield and one Nightforce focused at about 400 yards (estimated) when set to infinity and a bit short of the two closer ranges. One Nightforce scope focused sharply at 200 yards when set to infinity and had to be set at about 100 yards to focus sharply at 200 yards. All of these errors are caused by sliding tubes holding lenses moving in fixed camming tubes each holding lenses with slightly different focal lengths. And there's a tiny amount of slop in the fit of the inner tube to the outer tube as well as the inner tube's cam pins in the outer tube's curved slots they slide in.

The scope company's optical engineers calculate an average focal length for each of the three groups used (two of which are in the zoom part) to focus the target on the reticule. With a small percent spread in focal lengths of each lens groups, a given amount of movement by the power ring ends up making their combined focal length different and that puts the image focused either behind or in front of the reticule for a given range and power setting.
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Last edited by Bart B.; November 6, 2013 at 08:28 PM.
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Old November 6, 2013, 09:11 PM   #20
FLChinook
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Pass on a Variable Power Scope if Accuracy is Needed

This finding is very distressing indeed. You did not mention Zeiss nor Swarovski but the principles you list will apply to them as well.

Let me ask for a personal recommendation... also for any others who are following this thread:

"What scope do you use for a rifle where the shooter is a stickler for wanting accuracy and does everything possible to achieve it and where the ranges mostly used are 100 to 300 yards (targets and deer sized animals).

Thanks
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Old November 6, 2013, 09:38 PM   #21
Bart B.
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Weaver Grand Slam 2-8X with reticule of your choice.
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Old November 6, 2013, 10:44 PM   #22
FLChinook
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Quote:
Weaver Grand Slam 2-8X with reticule of your choice.
... and if you were going to go to a fixed power...?
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Old November 7, 2013, 12:52 PM   #23
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Fixed powers do not have this issue. They are not adjustable in power settings
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Old November 7, 2013, 01:43 PM   #24
Bart B.
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Quote:
Something that could be a factor:If the rings are not truly co-axial,there will be some tube distortion when the rings are tightened.
I don't think so.

If the scope tube's clamped tight in rings, it doesn't matter how much distortion the tube has changing the optical alignment of the lens groups that might be caused by the ring's axis and alignment with each other. It's repeatable from shot to shot and doesn't change by ajusting anything; elevation, windage, power of focus things. Ring mechanical axis and centering with each other perfectly only happens when both rings are first installed on the bases then lapped with the right size tool to 100% exact diameter of the scope tube. Or, with adjustable ring sections that first align with the scope tube then on the tube's axis in them as the rings are tightened on them.

To say nothing about what happens to aluminum scope tubes when rings are overtightened on them. Some folks have epoxied their scopes in rings to ensure the rings don't squeeze the inner mechanics rendering the scope towards bad stuff. Most epoxies will release their bonds to metal when frozen and changing rings on such a scope means it spends a day or two in your freezer.
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