Shifts in POI from changes in magnification?

[sambar]

A question to those in the know:

I'm familiar with the notion that in non-first focal plane scopes the POI will shift if using a magnification other than that at which the scope was zeroed.

What I'd like to know is the order of magnitude we're talking about.

Is there some formula / rule of thumb to work it out? That would great, so long as fairly simple.

If not, it may help if I give the particulars of my own case:

I am using a Bushnell Elite 6500 for hunting, which I zero at 160 yds, and use at the maximum 16x magnification whenever possible.

When used with the Archer NV add-on, however, I get a much brighter picture at 10x magnification. Therefore, if the shift in POI is not too great, I'd like to carry on with everything else at 16x, and just drop to 10x when using the add-on.

The normal range for me would be 50 - 100 yds, sometimes, but only when unavoidable, out to 150, with my longest ever shot being 175 yds when everything was perfect for taking the shot.

If all the variables are still too great, then I suppose a totally workable answer would be to know what would be the change at 100 yds if I dropped to 10x having zeroed at 16x.

Coincidentally the '10' digits on that scope are in red, whereas all the others are in black - anyone know what that's about?

Many Thanks in advance.

 

[rem700]

Hi

I think the RED (10) on the variable mag ring

is the mag ,that the ret can be read correctly ie mill dots correct size for ranging

Or 2nd shot correction

Hope that helps

ATB

[Breacher]

I have never had ANY shift in POI - Simmons Whitetail, Nightforce, IOR etc

 

As I understand it, on a decent scope poi of zero should remain constant regardless of magnification.

 

Are you confusing poi of zero shift with change in reticle subtensions ?

 

 

Although your zero should remain constant, if you use mildots or other parts of reticle for holdovers ( 2nd focal plane ) they will only be constant and correct size at ONE magnification - probably the one marked in red.

[brown dog]

POI shift on sfp scopes relates to the mechanical movement of the reticle that takes place when a sfp scope's power setting is changed.

As it relates to the degree of mechanical slop in the zoom assembly of each individual brand, or even from scope to scope within a brand, it is an entirely unpredictable variable. Your scope may be ok, it may not. The only way is to try it at the various powers, or to clamp the scope and look through it whilst zooming to see if the crosshair wanders as the power changes.

[The Gun Pimp]

This is one of the tests I always do when reviewing scopes - which are mostly SFP. I don't think I've every had one with a shift in impact point.

[brown dog]

I'd suggest, when discussing zero shift on high quality sfp scopes, rather than saying 'doesn't happen' it'd be more accurate (pun!) to say 'shouldn't happen, but could'

Individual mileages vary.

[ds1]

Zero shift in 2fp scopes will happen as a result of mechanical slop, and there always will be mechanical slop built in so that the scope continues to work under temperature chamges and don't bind - just a design limitation

[Grouse]

This is one of the tests I always do when reviewing scopes - which are mostly SFP. I don't think I've every had one with a shift in impact point.

I concur with you, always have tested, never had a shift.

[sambar]

Thank you gentlemen. I think that that flood of information answers my question nicely.

Good to know what the figures in red meant. And nice to think that at 10x it falls (purely coincidentally) at the best point for judging hold-over etc, since that is somehow less easy to judge than in full daylight.

An actual test on the range will always, of course, be the acid test, but I do like Brown Dog's idea of fixing on a target then changing the magnification - why didn't I think of that?

Demonstrates nicely the real value of a decent forum where we can draw on the experience,knowledge and common sense of a wide range of people.

Thanks again to all of you for giving me the benefit of yours.

[ds1]

Sambar, you can also test the poi changes with magnification and scope tracking using a bore sighter. It basically gives you a lined grid in front of your reticule. What a bore sighter does not do however is test for changes under recoil.

[gbal]

sambar,I think you summarise this forum well-here for contrast is post 11 on an air rifle forum (air rifles maybe more prone/use cheaper scopes etc)_ the OP was "Does adjusting mag zoom affect POI?";and here is the summary of replies:

"9 shooters

10 answers

All of them right!"

 

Well,not-the double answer shooter was wrong on both his assertions,too (point of aim mudddled).

 

OK, I have no quibble of substance with any of the replies here except that they don't all answer the question-indeed none indicate the degree of POI change if it occurs at all. Very few scopes are precise to the second decimal on every optical test!

 

SFP construction can allow a (small) error unless the construction is of high quality (FFP are immune,but cost more anyhow....)

The better (£300+?) scopes are now pretty good (Burris eg have a "posi lock' to lock the adjuster on the erector after zeroing.)

 

There are very large numbers of modest SFP scopes used by hunters-it's rare to see POI shift with mag reported. More impressive are the comments here by testers/users who have posted-Gun PImp,Breacher,Grouse...I'll add that I have over 30 scopes-mostly variable,all SFP-from relatively modest Tascos,to Leupolds and Night Force-dating from the seventies,and I've never experienced a POI/mag shift issue-it might have been there in the basic scopes,but I never noticed it in use,and was not there in the better scopes,which were Bench Rest tested.

As said,very few scopes are perfect-as most tests reveal,eg the clicks can be out a calibre on a round the box test-but that is very good (this is just as true for up market FFP as SFP scopes-mechanics are very,very good,not perfect. If it's slop,it's caviar and champagne slop!!

 

The data from fairly extensive current tests is in line with GP et al-Cal Zant on Precision Rife blog testing all the top use scopes in that discipline has never even commented on this issue-presumably has never found a flawed scope?

Earlier US tests -though a few in this century,note-have found some low end scopes out by maybe 1/2 moa....and a German hunting optics test found most were fine,but an occasional 1/2-3/4 moa shift in extremes.(Poor quality control?-warranty replacement).

 

There is then a small risk,of a modest POI shift in the worst scopes.

So check yours and see,which is what you decided to do.I'd think even 1/2 moa at 100y with NV is useable,even if not allowed for?.

 

(this size of POI is considerably less than typical with shifted positions-especially tested on bench,used in the field.....or 'holds' on a bipod etc....just to put it in some perspective-Tubb has done the research on this,among others).

 

POI shift is pretty well gone in better SFP scopes-just like 'far too thick reticles' in FFP scopes is history (it was much less an issue in Europe,as hunting shots were much shorter,usually).

Whatever,there is now far better choice.Vorspruch.

 

ten replies

all right

and figures too

is UKV made by Carlsberg?

 

:-)

gbal

[sambar]

Sambar, you can also test the poi changes with magnification and scope tracking using a bore sighter. It basically gives you a lined grid in front of your reticule. What a bore sighter does not do however is test for changes under recoil.

 

Good idea ds1.

Another case, like with brown dog's reply, of "Why couldn't I have thought of that for myself?".

I think sometimes one gets so tied up in one aspect of a matter that simplicity goes out of the window - certainly one of my shortcomings time and again. Guess that, for me, that is the real value of good mates / contacts / fellow forum members.

Thank you.

[sambar]

Well gbal, another very full answer, thank you - your answer was longer than my question! (And equally as well informed as your knowledge of the sex life of bigamous foxes).

Precise and thorough testing is of course the only way for me to know what applies in the case of my own rifle and scope. However I take confidence from the mass of evidence forthcoming here that, unless i have a cheap or faulty scope, the order of magnitude we are talking about is either non-existent or negligible.

The scope in question has served me very well for several years. It has held zero admirably and has always adjusted well, whenever I've had cause to re-zero it, so I'm confident that it's well engineered and have had confidence in it from the start.

The rifle is zeroed to give a 1 inch maximum point blank from 25 yds to 180 yds (according to the ballistics tables i've used). It is used off sticks in the field, so the single biggest limiting factor is my own (modest) shooting ability. The distances I shoot at too, for that reason, are also pretty modest,so if we're talking sub MOA, then that's more accurate than I am in those conditions.

Additionally I have never actually felt that changing magnification actually made any difference to my shooting in these circumstances - I still feel that success is down to how I shoot on the day rather than to my equipment - that's the importance of having gear that you're confident in, otherwise there are just too many variables.

So why then have I put everyone to all this trouble with this question if, as I just said, I've always felt it was down to me anyway? Well, the sad truth is that I've noticed that more and more recently, in order to get a decent sight picture, I've been having to turn down the magnification to give me a brighter picture. The question arose, and doubt then took seed in my mind. That can affect confidence, and without confidence even the best equipment won't have you shooting straight. I feel that is now addressed, and that I can move on.

Any optometrists out there on UKV?

[VarmLR]

As above, the bore sighter is a good way of doing it and the type that clamp to the end of the rifle have the advantage that it doesn't matter of the rifle moves because the grid won't move compared with the scope.

 

As well as the possibility of a slight POI shift with mag change on some scopes, there are other things possibly more to be concerned with for longer range shooting, especially where budget scopes are concerned. At least if there is a slight POI change with mag, you can be sure that dialling into a specific range and setting a specific mag will at least result in consistent POI from that point on all being equal. However, all is not equal and it pays to be mindful about other considerations.

 

One might be parallax error (in particular the movement of cheek hence eye position relative to the crosshairs between shots. On some scopes it is very noticeable just how any slight movement of the head results in the crosshairs moving on target, especially at higher mags); the repeatability of turrets, and the quality of mounts. I was quite surprised just how much it improved precision going from so-so budget mounts to a set of Teir Ones for example.

 

Each of these things can have a more dramatic impact on POI consistency than some slight change in POI with mag changes, but it's good to cover all bases and tackle each one at a time as it makes pinning down errors that much easier.

[sambar]

More good and salient points, thank you VarmLR.

I've decided, in view of how it's served me for a good time, that I'm happy with my gear - the scope mounts, as per your experience, were an important factor that I, fortunately, sorted out early on in my quest.

When you consider the myriad of tiny factors involved in attaining even workable accuracy, and the lengths you sometimes have to go to in order to sort out each individual one, you realise that it's a wonder we get to hit anything at any distance.

I'm very clear in my mind that, a bit like "standing on the shoulders of giants", we owe it to the efforts of those who passed before us - and also to our contemporaries who give unstintingly of the fruits of their own painstaking endeavours.

Why couldn't I have been interested in something nice and straightforward like stamp collecting?

[Hatzi]

Funnily enough the worst premium scope i have had in this respect was an FFP S&B PMII surprisingly....because it was plain knackered and for other reasons lol.

 

You get it marginally on a lot of cheap chinese scopes in 2fp and I have a Picatinny rail bolted to my bench to review this in seconds. I haul rlfles around carrying them by their scopes, bump them around and deliberately try to temp wander, if mounts cant handle using the scope as a carry handle they are useless under recoil. I tend to find a lot of cheaper scopes arc back and forth from zero position as the helix inside tracks back and forth so they are on at low and higg mag but kind of banana away through the range if they do drift then come back in. The biggest issue I have on scopes is when reticles wont focus sharply and I can think of two 'mid tange' brands I just wont bother using because of this. Occular lenses also have some major flaws that ruin otherwise mechanically sound scopes and this is becoming especially apparebt in digital Night vision and Thermal kit.

[Murph]

Sambar, you can also test the poi changes with magnification and scope tracking using a bore sighter. It basically gives you a lined grid in front of your reticule. What a bore sighter does not do however is test for changes under recoil.

Plus 1 on this. My local gunshop has a bore sighter. I bought an MTC taipan and wanted it bore sighted for my 22. I asked to adjust the magnification on the scope to see if there was any error. There was a slight bit of error on it. The owner of the shop told me that almost every scope in sfp under £1000 showed from slight to horrendous!. I tried my z6i 5-30x50 on my rifle to see what it was like. There was no descernable movement in poi through the whole zoom range.