Parallax and POI

Discussion in 'Hunter (HFT) & Field Target (FT)' started by Kingplinker, Oct 3, 2009.

  1. Kingplinker

    Kingplinker " Horsham HFT champion 2010 "... apparently...

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    Hi all

    Was just sent a link from another forum wrt the above .

    A few weeks back I told a few members of the club that changing PX can alter POI , I was then told thats not the case in actual fact I was told that the only way POI would change would be if you changed mag .
    :rolleyes:
    ............ I'm right aren't I :D:D:D

    Dave
     
  2. AndyJ

    AndyJ Member

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    Parallax SHOULD'NT change poi but on cheaper scopes it could alter it. Parallax only affects poi when your eye position is wrong causing you to get error
     
  3. Dave Ramshead

    Dave Ramshead England HFT Team squatter

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    I was loaned a lightstream to try, zeroed it at 22y with 22y px - pellet on pellet

    Turned px to 40y and tried grouping at 22y again and it had moved the the zero / poi out by 15mm to the right.
    move px back to 22y and it was back on the cross.

    So, spending more money on the scope doesn't mean you will buy less px error
     
  4. RobF

    RobF Administrator Staff Member

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    As I pointed out over there...

    If you have a scope which is not correctly PA'd to the range your shooting at, even if you have consistent head position, it's possible that you could be shooting consistently with error, ie constantly offsetting the reticule from it's correct alignment... so by dialing in PA correction, you could then see a shift, as now the crosshairs cant move across the target, where as before they could...

    hope that kind of makes sense.

    In short, PA correction doesn't allow the reticule to wander across the image when it's dialled in. If your scope is not dialled in, you could see either inconsistent or wide groups, you could see a shift in POI but with a tight group (in which case your head is consistent but not aligned), or, if you are perfectly aligned and consistent you could see nothing unusual, but dialing out the PA error cures all.

    It could be the reason why some very good shooters see shots wander in HFT, because getting that eye in the same place and aligned in all positions can be very difficult. Bobbing and weaving the head (assuming the stock/postion) allows you to determine if PA error is present to start with, and whether you are at the extremes of the crosshair movement or somewhat central.
     
  5. AndyJ

    AndyJ Member

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    Yeah sorry didnt explain as well as i could, what i meant was parallax is error caused by scope/eye position, where as altering the zoom actually alters the poi(not zero) as opposed to parallax where it is a percieved poi change due.

    And that was why i put "shouldn't" change poi but as we know manufacturing tolerances can play a part as the lens may not move perfectly in the same plane therefore moving poi
     
    Last edited: Oct 3, 2009
  6. Jon

    Jon Member

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    Hence why Leupold call it focus and not parallax. As of yet fixed in rigid vices focus on full chat either way at an image never seen any decent Leup change poi by moving focus or zoom. Even going to extremes with head left, right or above as long as ret can be seen, its spot on.
     
  7. Ratgunner

    Ratgunner New Member

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    Ratgunner

    In all fairness to everyone I may be having a senior moment here, But!

    I've always understood parallax as the vision gained by viewing an object from two different angles?

    A view through your scope should be free from distortion, shadowing, or graying out, ergo no parallax? or vision/scope fault?

    A scope set to 35yds or 350yds shows clear focus, then these are found ranges by range finder?

    If focussing your reticules requires a large adjustment then your range finder is now incorrect as the scope values have shifted?

    These are corrected by pacing/measuring ranges and marking range finder accordingly?

    I am more than happy to be shown the error of my ways,

    Soon to be sixty five years stupid.
     
  8. D Martin

    D Martin Non member

    So is incorrect eye inlinement and parallax error the same thing ?

    My son shoots his SR6 with parallax set at 40ish yards as he can't focus on long shots if set any lower and on 10-12 mag he was getting loads of error yet we dropped it down to x8 and now the error has gone and everything is clear from 12-15 yards all the way out.

    My EB however is set at 25 metres I believe on the EB and is clear for me from very close to 55 yards and I suffer very little in the way of parallax/eye inlinement error and adjustting the parallax doesn't seem to alter the poi, though I do try to make sure that my eye is directly behind the lens before every shot.

    This may explain why some shooters miss high,low,left or right when taking kneelers and standers as their head is in a different position to when they are shooting prone, unless like me they are just crap:D

    So is parallax error and eye inlinement the same thing ?

    Dave
     
  9. JAGXKRS

    JAGXKRS Sparky's Bodyguard

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    Think so mate, I know if I my head/eye is right and the range is right its down with the 6500. If my eye is wrong on scope I can miss:(
     
  10. RobF

    RobF Administrator Staff Member

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    Ok, just been through this in a demo.

    Parallax error. Caused by the thing you are measuring with not being in the same plane as the thing you are measuring... ie if you drive along at 70, your passenger might see 75 because the speedo needle is not sitting on the surface of the gauge but perhaps 5mm above it... so if you shift your head, the reading changes. It's why rulers are thin, so they measure what they lay on, not allowing for any parallax error with them not being close to what they are measuring.

    In scopes, the image is pulled in from the front lens and focused at a point. Like cameras, scopes can only focus to a single point at a single range. You eyepiece is focused on the reticule, which ideally should be at this point. However, because scopes focus at a single point for a single range, this focus point may not be exactly in the same plane as the reticule... it may be a smidge forward or behind... not such a big deal, except you are looking at it at perhaps 10x or more, so the error by these two moving around when your head moves (in the same way as the needle on a speedo does), is magnified. What also is magnified by magnification, is that you might notice it's not so in focus. Which is why the FT mob use high mag, so they can see when the scope is perfectly in focus, and if you took the time to focus the reticule beforehand, when the two are perfectly in focus, you will have minimised the PA error to nothing, or very close.

    Obviously the reverse is true, PA error is less noticable on low mage scopes, where the apparent movement is reduced and the focus looks good. However the PA error is still present if it's not dialled out.

    Assuming you have focused the eyepiece to the reticule first, sharp focus is a by product of low parallax error, but if you have a scope with a wide focus range (ie a small objective, low mag scope) be aware that your focus may look sharp for a wide range of distances and consequently you may still have PA error. But really, unless you have a high mag scope, one good way to test is to move your head around... if the crosshairs move across the target, it's not PA'd to that range.

    I cant remember which is worse, a far focused scope being used at close range, or a close focused scope being used further out... you might want to play with that a but.

    Also, if you have a scope that can resolve detail greater than your eye can, you may not get on well with it. Having astigmatisms, my eye goes in and out of focus, so i prefer scopes with definite focus, rather than something like a leup which can resolve really good detail that my eye won't hold on because it's focus wanders. Past a certain point, there is no benefit to me in seeing the nth detail, because my eye won't hold it.

    I think i need to write this up with some pics... but work is mad at the moment, so don't hold your breath :D
     
  11. JAGXKRS

    JAGXKRS Sparky's Bodyguard

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    I think you just said if you bounce your head around behind the scope your knackered:D:D:D
     
  12. D Martin

    D Martin Non member

    That kinda made sense, thanks Rob.

    Dave
     
  13. martin c

    martin c New Member

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    great post rob

    well explained i think i am starting to get it at last :rolleyes:

    [feels like a sticky to me]
     
  14. Ratgunner

    Ratgunner New Member

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    Ratgunner

    :)
    Thanks for the above,

    I had not taken astigmatism into account, and also you put the subject so much better,

    I am partially sighted in my left eye due to a small stroke,
    stupidly in spite of this my left is far more cognizant of detail than my right eye?? this was reaffirmed recently on my yearly Retinopathy (posh eye test)

    For the benefit of all our shooters would not a database of such problems be usefull to our members?
    as would be the solutions to said problems as noted by our Members?
     
  15. nyamazan

    nyamazan New Member

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    Robf,

    I was always lead to believe Parallax error was less at low mag. From your post you say it's less NOTICABLE. Do you mean the error is exactly the same as in high mag?
     
  16. RobF

    RobF Administrator Staff Member

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    I'm not sure to be honest... i'll have a look next time i'm down the range. I'm sure there's some maths behind it, and essentially the planes of focus are the same distance apart, so my guess the error would be the same, but less noticable... but will check.
     

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