How common is POI shift with AO adjustment?

[vigilandy]

Still have not had time to sort this out!  For now I'm keeping the AO set to 35 yards so it's pretty decent focus give or take 15 yards.  Hawke Varmint 6-24X44 and a Leapers 3-12x44 are in hand,  but still need time to mount and zero.  A grid bore sighter would make my life a little easier since my free time is usually after its too dark to shoot. 

@sscoyote -  got a link to that model? 

[Methuselah]

Still have not had time to sort this out!  For now I'm keeping the AO set to 35 yards so it's pretty decent focus give or take 15 yards.  Hawke Varmint 6-24X44 and a Leapers 3-12x44 are in hand,  but still need time to mount and zero.  A grid bore sighter would make my life a little easier since my free time is usually after its too dark to shoot. 

@sscoyote -  got a link to that model?

Andy,

You do understand the main feature of an AO scope is that turning the AO to the right spot eliminates parallax?  Leaving it at any one range and shooting another means unless you are practiced enough to have your cheek weld in exactly the same spot you zeroed, you are introducing error that can be avoided by adjusting the AO (don't have to head-bob, but it will squeeze more accuracy out of your shooting setup if you do).

Try this, pick an intermediate range, i.e. between 20 and 30 yards, set a ruler there.  At maximum power setting, start with the focus distance setting well under the target and slowly adjust it up until it just focuses.  Do the head-bob and note how much your cross hairs move along the ruler (write it down).

Then, turn it higher until it goes out of focus and slowly turn it back until it just focuses, then do the same thing --- write down the error.

Whichever number you just wrote down that is higher, double it.

This is your worst case shot group CTC if were able to shoot a perfect one-hole group at that distance.  So, for example ignoring windage, at 25 yards the match of your skills, the accuracy of the rifle, the consistency of the pellet choice all come together to give you a normal 3/4" group.  Now lets say you wrote down 3/4" above.  When you shoot this distance your group could wind up being 1 1/2" CTC due to parallax error.  HOWEVER, if you do the head-bob, it should be 3/4".  NOW, IF YOU LEAVE IT AT THE 35 SETTING YOU SPOKE OF, AT 25 YARDS IT MIGHT INTRODUCE A MAXIMUM 2" PARALLAX ERROR AND THAT SAME GROUP THAT WOULD HAVE BEEN 3/4" CAN NOW BECOME A 2 3/4" GROUP.

If you are not interested in adjusting the AO and plan on using the rifle for hunting, I suggest using iron sights ... no offense intended - I just think it would work out better for you?

Also, the gadget/toy will not do anything for you that you cannot do (probably more accurately) by walking off your shooting range 5 yards at a time and doing the head bob at every point then writing down the POA errors to use for corrections (which can be put on a tape over the scope numbers).  There's more to it than that, but my guess is that gadget is worthless if you plan on leaving that AO dial on 35...

If I got any (all?) of this wrong, hopefully someone will correct me!

HTH
Steve

[sscoyote]

Yessir, here it is--



this is the magnetic one, and the one I use. Other companies make them too, but I think this is the cheapest one. This sucker saves me time and often frustration at the range. Recently I mounted an optic on a pistol and found that the optic would not zero horizontally with the mount system I had...but I found it out before I wasted the gas/frustration to go to the range. It is especially useful for detecting reticle shifting for whatever reason, and you can do it in about 30 seconds, and you will know, without question, if there's a problem before you even fire a shot...nice to know, IMO. I recently thought a particular "long-range" optic had way more than 100 MOA of turret adjustment, and was all happy until I mounted it on a gun and started turning turrets to check reticle movement. Guess what, the turret kept turning but no reticle movement after something like 90 MOA or so--good to know...sometimes. I use it every time I mount an optic.

Yeah it's a "gizmo"...but it's one of the most useful tools (AKA gizmos) I've ever used.

[robertr]

BOTH rifles sport the same scope.   I got a replacement for one on Friday and had a chance to shoot today. Same problem!  I set a target at 40 yards,  set the AO using head Bob method,  and zeroed the scope.  Easy 3/4 inch groups without even trying. Tiny groups are possible with this rifle,  but that wasn't the goal today.    Have I mentioned that I love this gun? 

I then changed the AO to 25 yards  and shot again.  Still tiny groups but POI shifted up and to the right about 1.5 inches.  Returning the AO to previous setting put the POI back at original location.  I then moved myself to with 25 yards of the target.  With AO still at 40 yards,  I shot high which is logical.  Then adjusted AO to 25 yards and confirmed with head bob that I was parallax free.  Pellet went even higher and off to the right again about an inch. 

  Since you zeroed the scope at 40 yards it would shoot high at 25 but should stay zeroed left to right, if it moved to the right after setting parralax to 25 yards shooting at 25 yards it could be a scope problem.
 I have an old Tasco air rifle scope with no parralax adjustment except for taking off the lock ring on the objective and adjusting it that way. I checked the accuracy of the paralax setting using a grid type bore sighter, I turned the objective ring while looking through the scope and noticed the reticle would move in a circle pattern, it should normally just move up or down slightly.
 So now I just find a happy medium and make sure I hold consistently. Hope that helps.

 I use the grid type sighter the same way sscoyote explained.

[vigilandy]

@ Methuselah - The only iron sites that get any use are the ones on my Blue Streak, and I'm no good with it unless the target is pretty much point blank.      I normally use the AO quite frequently!  It's just that this particular scope doesn't stay zeroed when I adjust the AO.  My current strategy to zero the scope at 35 yards which seems to be the best compromise for me in terms of parallax error.  I understand that POI should shift up and down depending on distance from target and where the scope is zeroed.  The problem with this scope is that the POI wanders by much more than you could expect when the target is moved and AO adjusted accordingly.   

Try this, pick an intermediate range, i.e. between 20 and 30 yards, set a ruler there.  At maximum power setting, start with the focus distance setting well under the target and slowly adjust it up until it just focuses.  Do the head-bob and note how much your cross hairs move along the ruler (write it down).

Then, turn it higher until it goes out of focus and slowly turn it back until it just focuses, then do the same thing --- write down the error.

Whichever number you just wrote down that is higher, double it.

This is your worst case shot group CTC if were able to shoot a perfect one-hole group at that distance. 

I've never thought to try to focus "forward" and then "backward" before.  Makes sense now you mention it. I will have to try this to see how much a difference there is on the scope.

@robertr - I think we are using the same strategy for under-performing scopes...   

@sscoyote - Thank you! The grid bore sighter on order now.  This should allow me to do a little bit of work after the kids go to bed.  By then, it's too dark to shoot targets. 

[vigilandy]

Well,  I got my Bushnell boresighter.  Not sure how use it as a boresighter, but it sure is useful for figuring out scope problems!   I've never noticed the side to side play on the AO ring until now.  It's only a small movement but it turns out that the crosshairs jump by quite a large amount regardless of whether I'm actually making adjustments!   

[Methuselah]

  Since you zeroed the scope at 40 yards it would shoot high at 25 but should stay zeroed left to right, if it moved to the right after setting parralax to 25 yards shooting at 25 yards it could be a scope problem.

Or just scope cant.

[Methuselah]

I have an old Tasco air rifle scope with no parallax adjustment except for taking off the lock ring on the objective and adjusting it that way. I checked the accuracy of the paralax setting using a grid type bore sighter, I turned the objective ring while looking through the scope and noticed the reticule would move in a circle pattern, it should normally just move up or down slightly.
 So now I just find a happy medium and make sure I hold consistently. Hope that helps.

Parallax is the difference in two images lining up with your eye.  Unless reticule and target are EXACTLY in the same focal plane moving your eye changes how they line up (parallax error).  Isn't a loose front lens while in the process of adjustment invalid?

Also, gizmo ... I don't understand how you can establish a valid relation to a 25 (or 40) yard target looking at something inches away.  Maybe I'm missing something but if parallax is eliminated all that means is the cross hairs are not lying to you about where your line of sight meets the target.

[Methuselah]

@ Methuselah - ...

The problem with this scope is that the POI wanders by much more than you could expect when the target is moved and AO adjusted accordingly.

Maybe scope cant?

All I know is parallax is a measurable thing.  If you do the head bob and the cross hairs stay exactly on the POA and you hit what you just aimed at, then if you change AO setting you should be able to repeat what you just did over and over again whenever you come back to this same range (barring small uncontrollable issues like the effects of temperature and barometer influence).

If this doesn't get you hitting the POA at different ranges I'd have to think that through, but we might need more information to figure out what is going on ... it can get complicated, sometimes its simpler to gather info by trying iron sight shooting or a second scope...

I hope I haven't botched the the parallax explanation!

[vigilandy]

Possible scope cant is an independent factor in my problems, but I can't confirm that without spending some more time shooting.  I can tell you though I an see the crosshairs physically move when I put sideways pressure on the focus ring.  Spinning the ring will cause the reticle to move in a circle.   

I have three of these scopes and two do the same thing (crosshairs move in a circle when using AO, sideways movement with I push and pull the AO ring sideways).  The third one is much better and barely moves compared to the other two.  Will have to find time to test the "good" one.

[Methuselah]

Possible scope cant is an independent factor in my problems, but I can't confirm that without spending some more time shooting.  I can tell you though I an see the crosshairs physically move when I put sideways pressure on the focus ring.  Spinning the ring will cause the reticule to move in a circle.   

I have three of these scopes and two do the same thing (crosshairs move in a circle when using AO, sideways movement with I push and pull the AO ring sideways).  The third one is much better and barely moves compared to the other two.  Will have to find time to test the "good" one.

I can't imagine anything accessible to the outside of the scope that you can turn where the crosshairs will turn too.  Wish I had it in my hand in case it's a communication issue but just hearing that kind of thing for the first time in the same sentence as you saying two of them do it ... well, stealing a line from Star Trek, "that does not compute".

Maybe a locking screw came loose in the barrel somewhere, but I'm completely without idea ... hope someone else caught what I'm missing!

[sscoyote]

Steve, if you think about it the only thing the parallax adjustment does is that it puts the x-hair and image on the same focal plane. It doesn't tell you if the zero has changed WITH INTERNAL MOVEMENT OF THE OPTIC. Worked with an optic awhile back that changes 5 MOA from 5 yds. to infinity horizontally. Now that's not that big of a deal since 5 MOA at 5 yds. is .25" but some might think so. Wanted to know recently what my 4-12X Burris Mini was doing with parallax and power changes since I hadn't tested it. Slapped the BGBS on the rig it's mounted on just yesterday and sure enough all was fine. There was no optical center movement with internal movement changes. Took all of <5 minutes, no gas, significant time or bullet investment.

[vigilandy]

I can't imagine anything accessible to the outside of the scope that you can turn where the crosshairs will turn too.  Wish I had it in my hand in case it's a communication issue but just hearing that kind of thing for the first time in the same sentence as you saying two of them do it ... well, stealing a line from Star Trek, "that does not compute".

Maybe a locking screw came loose in the barrel somewhere, but I'm completely without idea ... hope someone else caught what I'm missing!

I'm not sure how the scope works internally,  but I can literally see the crosshairs wandering on the target grid (on the boresighter) when I turn the AO.

[mobilehomer]

The focal point of a lens is at the end of the focal cone. If the lens is not perfectly mounted that point will move. Also, the nature of threads is that there should be some play so that they can be moved, not interference fit. This allows more movement in the lens. I have read accounts of ALL price and quality ranges of scopes doing this. I would also thing that side focus scopes are less susceptible to this, but not sure.

[sscoyote]

The focal point of a lens is at the end of the focal cone. If the lens is not perfectly mounted that point will move. Also, the nature of threads is that there should be some play so that they can be moved, not interference fit. This allows more movement in the lens. I have read accounts of ALL price and quality ranges of scopes doing this. I would also thing that side focus scopes are less susceptible to this, but not sure.

I have also read that AO scopes are more prone to this too, though my Burris as mentioned earlier doesn't have this problem. Awhile back, there was a discussion about this problem on the Pyramid Air blog site, and one of the fellows there ClaineB I think his handle was, had the same experience that I've had. He said that 2 companies listened to him regarding this problem whereas most of the others blew him (the phenomenon actually) off--Vortex and Leapers. Lo and behold, the one Vortex I checked had no problem. I've yet to check my Leapers, but intend to soon. The amount of movement is quantifiable, so my suggestion to the original poster is see if the zero movement is the same as what he gets with the BS'r (no pun intended). IT MUST BE REMEMBERED THOUGH THAT THE RETICLE "MOVEMENT" SHOULD NOT BE MISTAKEN FOR THE "MOVEMENT" OF RETICLE SUBTENSION CHANGE WITH POWER CHANGE THAT'S CHARACTERISTIC OF SFP SCOPES. Watch for reticle axis movement only!

[Methuselah]

I can't imagine anything accessible to the outside of the scope that you can turn where the crosshairs will turn too.  Wish I had it in my hand in case it's a communication issue but just hearing that kind of thing for the first time in the same sentence as you saying two of them do it ... well, stealing a line from Star Trek, "that does not compute".

Maybe a locking screw came loose in the barrel somewhere, but I'm completely without idea ... hope someone else caught what I'm missing!

I'm not sure how the scope works internally,  but I can literally see the crosshairs wandering on the target grid (on the boresighter) when I turn the AO.

Before it was rotating, now wondering?

I have three of these scopes and two do the same thing (crosshairs move in a circle when using AO, sideways movement with I push and pull the AO ring sideways)

Make up my mind  , there is a huge difference in the relation between those motions and the mechanical implications!

All things explained as they are to this point, the only advice I can give you that I am 100% sure cannot steer you in the wrong direction at this point is, 1) check all screws beginning AND end of session, 2) do head-bob ANY AND ALL changes in range, and, 3) don't let your gizmo dictate any scope adjustments while troubleshooting - use only the center of reasonably good shot groups for that.

I'd like to think if you retry that way and let us know what happens someone here can help ya figure out whats going on   

[vigilandy]

By wandering,  I mean wandering in a circular pattern!  Basically I center the crosshairs on the boresighter grid,  and then without moving my head,  I start turning the AO.  I can literally see the crosshairs moving in a circle.  Hopefully I've explained it clearly this time?

Thanks for all the input Steve.  I'll contact the warranty center and report back.

[Methuselah]

By wandering,  I mean wandering in a circular pattern!  Basically I center the crosshairs on the boresighter grid,  and then without moving my head,  I start turning the AO.  I can literally see the crosshairs moving in a circle.  Hopefully I've explained it clearly this time?

Thanks for all the input Steve.  I'll contact the warranty center and report back.

I guess a picture is worth 1000 words was never truer ... I suspect you mean the crosshairs stay up-down, left-right and the whole crosshair moves in that circle.  Sounds OK when you consider the threads have to have play and your looking inches away at the boresighter.  Or your saying the crosshairs spin like a propeller?!

If former, gotta be parallax, cant, or two bad scopes?

[mobilehomer]

Here is a picture of rifle scope optics. The point of the cone where it says "first image plane" is what I meant in my previous post. If the front lens is not perfectly square to the axis, or the objective wobbles on the bell, it will appear to move the reticle. The reticle IS NOT moving, the IMAGE is moving. Same effect, though, POI changes with AO adjustment. I hope this helps.

[vigilandy]

Thank you Ken.  That is a great diagram! 

Now I understand why the crosshairs appear to move.