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POI shift with magnification change in second focal plane scopes? Have you seen it? Login/Join 
Freethinker
Picture of sigfreund
posted
A Wikipedia article states,

“Variable-power telescopic sights with SFP [second focal plane] reticles can have slight point-of-impact shifts through their magnification range, caused by the positioning of the reticle in the mechanical zoom mechanism in the rear part of the telescopic sight. … Zeiss … get around impermissible impact shifts by laboriously hand-adjusting every military grade telescopic sight.”


Possible POI shift when changing magnification settings is something I read about variable power scopesights long ago, and it probably referred to the then-common SFP scopes of the era. I have not seen that claim in recent times, however, and am curious whether it’s still true of SFP variables. What say the membership?

https://en.wikipedia.org/wiki/...ght#Mil-dot_reticles

Edited: I changed “Does it occur?” to “Have you seen it?” because further searching turned up many references to the phenomenon. I was still surprised that I hadn’t run across references to in in a long time, but I suppose that the shift is either very minor or does not occur in higher tier scopes. Also, all the sights I’ve purchased in recent years have first focal plane reticles.

I am, however, still interested in any comments here.




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 42581 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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I’ve read there can be ‘some’ shift when one adjusts power. I would expect a higher potential with cheap scopes. I’ve never tried to analyze & test the issue.

One thing I do, is to try to do my final sight check near the magnification I’m likely to hunt with. Much of the time that could be 4-6 pwr range. Does that make a difference with an 80 yard shot at a deer? I’d wager no.
 
Posts: 4370 | Location: WI | Registered: February 29, 2012Reply With QuoteReport This Post
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Of course it's a thing.

For example, my Nightforce NXS 12-42X56 would happily shit the POI by about 1.5 inch at 50 yards going from 42X to 12X. I could reproduce that at will with a very accurate 22LR. It was uncanny.

As I explained in my stickied thread, this zoom shift occurs a lot but most people do not see it. This is one of the challenges facing the folks who make a dual reticle, as I explained earlier. The zoom shift will be instantly noticeable if your glass isn't "perfect."
 
Posts: 3162 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
Freethinker
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quote:
Originally posted by NikonUser:
As I explained in my stickied thread ....


Yes, you did. Perhaps I got distracted while reading and bleeped over that part. Thanks.




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 42581 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
Freethinker
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Considering all the things that get discussed by shooters, from the serious to the ridiculous, I am surprised to say the least that this discussion isn’t something that was lifted from an article in a 1976 issue of Guns and Ammo. People whine about the inside of their pistol’s slide not being finished and yet wandering POI is no big deal—?

I suppose if it’s small enough it has little if any practical effect, but still …, and the things one never knew.

I see that Nightforce has introduced a couple of new second focal plane scopes, and I wonder if their offerings have the same problem (other than the one already mentioned).




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 42581 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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I think POI shift could happen a lot more than people think and they would never notice it.

It begs the question "How much do you change the magnification of your variable scope?"

When hunting I typically leave it on the lowest magnification while stalking and crank it up to max if I'm setting up for a long shot. Shooting off the bench I usually use the Highest magnification. What do you guys do? NikonUser, in matches do you adjust the magnification more to see the mirage better than you would while hunting?

Reading the thread makes me wonder how much shift I might get and would I even notice it if it did because of the way I hunt.


Remember, this is all supposed to be for fun...................
 
Posts: 3906 | Registered: April 06, 2007Reply With QuoteReport This Post
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good thread -- side question somewhat related ...

when dialing in a LPVO... what magnification is the best choice to use for 'zeroing'??

given that we are admitting that POI changes at various magnifications... should you zero the scope set to the magnification we perceive to be the most frequently used ??

optics is a deep rabbit hole

--------------------------------


Proverbs 27:17 - As iron sharpens iron, so one man sharpens another.
 
Posts: 8096 | Location: Florida | Registered: September 20, 2004Reply With QuoteReport This Post
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I believe the shift occurs because the zoom lenses are twisted when zooming occurs and any slight imperfection can cause a shift of the image in relation to the second focal plane where the SFP reticle is found. It's not that the reticle moves; it's the image that moves while the reticle remains stationary. In an FFP design, the image of the FFP can still move due to imperfections in the zoom lenses, but since the reticle is already part of the shifting image, it moves with it.

March has a scope with a zoom eyepiece that is prized by benchresters. It's an SFP design with a fine reticle, but it's fixed focus. The zoom lenses do not move, they are fixed at maximum and stay there. The eyepiece is the one that has a small zoom and what you are zooming on is the image formed at the second focal plane. The drawback of that design is the small zoom range (it's a 40-60X, so 1.5X zoom) and the eyepiece extends as you zoom in. BR shooters have complete confidence there is no shift. This is also why you see a lot of fixed power scopes used by benchresters.

In answer to djpaintles's question, I never adjust the magnification on my scope. I ran my NXS at 40X all the time, except on days with bad mirage where I would crank down to about 30X and leave it there for the day. With my March-X 5-50X56 with ED glass, I kept it at 40X all the time, mirage or no mirage. Now with my March-X 10-60X56 HM, I am running at 50X all the time now, regardless of mirage.

I should think that people with lesser quality glass are forced to zoom down when the mirage is bad and they may also experience POI drift, but they just blame it on the mirage or other things.

I would think you need instruments to check this phenomenon, or luck into it as I did when I started using the NXS at 50 yards with that 22LR.
 
Posts: 3162 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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quote:
Originally posted by Sig209:
good thread -- side question somewhat related ...

when dialing in a LPVO... what magnification is the best choice to use for 'zeroing'??

given that we are admitting that POI changes at various magnifications... should you zero the scope set to the magnification we perceive to be the most frequently used ??

optics is a deep rabbit hole

--------------------------------


Most of my optics outside of Long Range competition are LPVOs, of some type. 3-9X, 2.5-10X, 2-7X, etc. I always zero these scopes at the maximum magnification. The reasoning is that would be the magnification I would use for utmost precision, low power would be used for speed where pinpoint accuracy is not as critical.
 
Posts: 3162 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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On second focal plane optics there is usually a “ sweet spot” in the middle of the zoom range that should be used for zeroing. Anything higher or lower might not allow the scope to retain zero at a given distance when the power is adjusted.
 
Posts: 2689 | Location: Finally free in AZ! | Registered: February 14, 2003Reply With QuoteReport This Post
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Another good reason to prefer FFP scopes, as I do now with the advent of complex ballistic reticles. I have SFP scopes on my rimfire bolt guns semi-autos but they are mostly just fine crosshairs. I tend to zero and shoot those on max magnification because it helps reduce user error...

But it would be interesting to check this sometime.
 
Posts: 2245 | Location: Indiana | Registered: December 28, 2004Reply With QuoteReport This Post
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quote:
Originally posted by Lefty Sig:
Another good reason to prefer FFP scopes, as I do now with the advent of complex ballistic reticles. I have SFP scopes on my rimfire bolt guns semi-autos but they are mostly just fine crosshairs. I tend to zero and shoot those on max magnification because it helps reduce user error...

But it would be interesting to check this sometime.


It's not a very good reason.

I have been thinking about this question since I first answered it here and I think I may have to modify my response regarding the FFP reticles.

Follow me on this and tell me where I am wrong. A riflescope is a aiming sight on top of a rifle. When we zero a scope on a rifle, we effectively point the aiming sight at the target and at those settings, the bullet will impact the target. That is the theory. Going further, the aiming point of the riflescope has a specific relationship to the bore and the muzzle of the barrel for that distance. If we remove the riflescope and reattach it but not quite the exact same way, the aiming point will be at a different place than the where the bullet will hit at the same distance as before. We didn't change anything internally on the scope, we just didn't reattach it in exactly the same way.

Now, if I twist the zoom lenses in the erector assembly and the FFP image is moved just a bit, that means the specific relationship between the aiming point and the bore and muzzle of the target is changed also. It would be the equivalent of moving the reticle away from the established zero.

So I think I want to change my answer and say that either type of reticle is subject to zoom shifting.

Of course, I could be wrong.
 
Posts: 3162 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
Freethinker
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You may be correct, NikonUser, but the independent information/consensus I found when researching the question myself was that FFP reticles are not subject to POI shift when changing magnification. I don’t recall the details of the explanation, but could it be that the reticle itself does not move in FFP scopes, and only the magnification changes? I obviously don’t know myself, but as I say I did look around some and what I do remember, I believe, was that the FFP reticle doesn’t move.

Added: This is a post on the Sniper’s Hide forum by “Mile High Shooting”:

“Lesser quality scopes can and sometimes do have some between power deviation as it takes time and money to ensure it doesn't take place. The deviation comes from the erector assembly where the glass is moving back and forth to create the variable magnification. If the erector assembly isn't perfectly manufactured and the lenses indexed you'll get the deviation at different magnification. This is only an issue in SFP scopes because the reticle is behind the erector assembly which allows for the misalignment. FFP optics have the reticle in front of the erector assembly so it is independent of the erector assembly positioning in regards to any potential deviation between magnification.” [Emphasis added.]

LINK:
https://www.snipershide.com/sh...fp-poi-shift.156523/

I did find a different article that said SFP reticles were just more susceptible to the problem than FFP reticles, but that it was still possible with FFP because they were attached to the erector assembly. Obviously someone is wrong, or at least wrong about some scopes and right about others.




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 42581 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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I will ignore the value-line optics I've shot in hunting rifles. Generally 2-7x through 3-9x. I've never tested them for pure accuracy at various magnification levels, and honestly it was never necessary. I'm not concerned with LPV optics.

My first precision scope was a SFP Nightforce NXS 5.5-22x. First used on a 308 bolt action, later rotated to a 6.5CM AR10, briefly on an AR15, now on a 223 bolt action. Once I got into the steel/precision/tactical match game, I realized SFP didn't play well with the need to shoot at lower magnifications. The scope/rifle was zeroed at 22x, but I shot more at 11x and doubled the reticle values for elevation and windage holds. My 100-yard zero at 22x was still spot on when the scope was set at 11x. Furthermore, I could not distinguish any POI shift out to 700-800 yards at 11x.

I had a SFP Leupold Mark 4 4.5-14x, which spent all its time on various AR15s. Zeroed at 100 yards at 14x. I often shot steel at various magnification levels from 8x-ish to 14x. Never saw any POI differences at the different magnification levels.

My first FFP scopes are Nightforce NXS 3.5-15x F1, and I now have three of them. Been on 6.5CM bolt action, 308 bolt action, 6.5CM AR10, and various AR15s. Shoot them at many magnifications -- generally 7-ish to 15x, but all the way down to 3.5x for close targets in a few multi-gun matches. Never saw any POI differences at the different magnifications.

Have one NF ATACR 4-16x F1. Dedicated to a precision AR15, zeroed at 16x, shot at all magnifications, never experienced POI shifts.

Have two NF ATACR 5-25x F1s. Been on 6.5CM bolt action, 308 bolt action, AR15 -- now on 6.5CM and 6CM AR10s. Zeroed at 25x, shot at all magnification levels, never experienced POI shifts.

Have two NF ATACR 7-35x F1s, on 6.5CM and 308 bolt actions. Zeroed at maybe 30x, shot at all magnification levels, never experienced POI shifts.

Have two NF SHV 4-14x F1s, on 22lr bolt action and 22lr AR upper. Zeroed at 14x at 50 yards, shot at all magnification levels, never experienced POI shifts.

Have a NF NSX Compact SFP 2.5-10x. Been on multiple AR15s. Zeroed at 10x at 100 yards. Primarily shot at 5-10x, never experienced POI shifts.

Have a FFP Vortex Viper PST II 2-10x. Been on multiple AR15s. Zeroed at 10x at 100 yards. Primarily shot at 5-10x, never experienced POI shifts.

Maybe I have good luck with optics. Maybe they have POI shifts at various magnification levels, but they are small enough that I can't detect them -- given my target types, ammo, and shooting skills. Every rifle system I have using the above-mentioned optics is capable of sub-MOA performance out to many hundreds of yards. I personally don't see POI shifts as an issue.
 
Posts: 6655 | Location: Colorado | Registered: January 26, 2008Reply With QuoteReport This Post
Freethinker
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Thanks, fritz, for all those examples.




“The fundamental cause of trouble in the world today is that the stupid are cocksure while the intelligent are full of doubt.”
— Bertrand Russell
 
Posts: 42581 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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quote:
Originally posted by sigfreund:
You may be correct, NikonUser, but the independent information/consensus I found when researching the question myself was that FFP reticles are not subject to POI shift when changing magnification. I don’t recall the details of the explanation, but could it be that the reticle itself does not move in FFP scopes, and only the magnification changes? I obviously don’t know myself, but as I say I did look around some and what I do remember, I believe, was that the FFP reticle doesn’t move.

Added: This is a post on the Sniper’s Hide forum by “Mile High Shooting”:

“Lesser quality scopes can and sometimes do have some between power deviation as it takes time and money to ensure it doesn't take place. The deviation comes from the erector assembly where the glass is moving back and forth to create the variable magnification. If the erector assembly isn't perfectly manufactured and the lenses indexed you'll get the deviation at different magnification. This is only an issue in SFP scopes because the reticle is behind the erector assembly which allows for the misalignment. FFP optics have the reticle in front of the erector assembly so it is independent of the erector assembly positioning in regards to any potential deviation between magnification.” [Emphasis added.]

LINK:
https://www.snipershide.com/sh...fp-poi-shift.156523/

I did find a different article that said SFP reticles were just more susceptible to the problem than FFP reticles, but that it was still possible with FFP because they were attached to the erector assembly. Obviously someone is wrong, or at least wrong about some scopes and right about others.


Let me just say that I had a nosebleed and a massive headache after I wrote my prior message. I think I may be overthinking this. Cool
 
Posts: 3162 | Location: Texas | Registered: June 20, 2012Reply With QuoteReport This Post
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