Parallax and magnification.

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  • Parallax and magnification.

    So I just noticed something for the first time today. Parallax doesn't seem to be a linear change across scope magnification, but a constant. What this means, is that it appears that apparent parallax is the same relative to the image in the scope, not to the target size. So basically, it's exaggerated at lower magnification while less apparent at greater magnification.

    Has anyone else noticed this, or is this this my imagination?

    I've been in the habit of shooting at the highest magnification, so I've never noticed it before.

    Edit: This should probably be in the technical forum, huh?
    Last edited by Guest; 07-15-2013, 03:04 AM.
  • LR1955
    Super Moderator
    • Mar 2011
    • 3355

    #2
    Originally posted by Rambozo View Post
    So I just noticed something for the first time today. Parallax doesn't seem to be a linear change across scope magnification, but a constant. What this means, is that it appears that apparent parallax is the same relative to the image in the scope, not to the target size. So basically, it's exaggerated at lower magnification while less apparent at greater magnification.

    Has anyone else noticed this, or is this this my imagination?

    I've been in the habit of shooting at the highest magnification, so I've never noticed it before.

    Edit: This should probably be in the technical forum, huh?
    RB:

    Just the opposite. It is more apparent at the higher magnifications.

    You may not have noticed much in terms of group sizes if you maintained the same location of your eye behind the scope for every shot.

    'Parallax free' scopes tend to be the lower magnifications because at lower magnifications you can't see as much detail so don't see as much of a difference.

    Higher magnifications are normally used to see better at longer ranges. And the farther out you go, the more dust, humidity, pollen, etc is in the air that distorts images. So, when you adjust your focus or parallax to get a crisper image, you are 'burning through' these obscurants. If the air was cleaner, less humidity, etc, your adjustments would be different.

    I don't think anyone could determine if the change is linear across the magnification range because you won't see as clearly on the lower magnifications as you will on the higher and parallax is adjusted based on how clearly you see things.

    LR1955

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    • #3
      LR,

      I always adjust parallax by wiggling my head around behind the scope before shooting. I've been in the habit of setting it and forgetting it though. That is, unless I changing range.

      Yesterday though, out of curiosity, I decided to do the jiggle test after powering the scope down. I noticed the parallax, and I noticed a relatively greater error across a greater portion of my target (smaller magnification, smaller target). When zoomed in at 32x the error seemed much finer relative to the target size. After adjusting parallax out at the greater magnification, it appears as if I have to adjust it again after moving to the lower magnification.

      It's almost as if the error is behaving as if it's second focal plane, and that it isn't simply a one setting fits all across the magnification range. Well, that's all based on my observation from yesterday alone. Perhaps it's a peculiarity of the less than stellar optic I was using? It was an old Tasco, I think.

      Comment

      • LR1955
        Super Moderator
        • Mar 2011
        • 3355

        #4
        Originally posted by Rambozo View Post
        LR,

        I always adjust parallax by wiggling my head around behind the scope before shooting. I've been in the habit of setting it and forgetting it though. That is, unless I changing range.

        Yesterday though, out of curiosity, I decided to do the jiggle test after powering the scope down. I noticed the parallax, and I noticed a relatively greater error across a greater portion of my target (smaller magnification, smaller target). When zoomed in at 32x the error seemed much finer relative to the target size. After adjusting parallax out at the greater magnification, it appears as if I have to adjust it again after moving to the lower magnification.

        It's almost as if the error is behaving as if it's second focal plane, and that it isn't simply a one setting fits all across the magnification range. Well, that's all based on my observation from yesterday alone. Perhaps it's a peculiarity of the less than stellar optic I was using? It was an old Tasco, I think.
        RB:

        Yes, the jiggling of the head behind the optic is a test to see if you have a parallax problem. I didn't mention it because most of my shooting with an optic is from a sling or a front rest using a toe bag to fine tune sight picture. With either of these positions, the head jiggle test doesn't work well so I maintain the best hold possible and adjust the focus to get a clear sight picture. I always give the head moving test a try but it works much better when you can rest the rifle completely without touching it.

        I don't have an answer for your situation because it is the opposite of what should and normally does happen.

        About the only things I can imagine are the ocular lens adjustment is not right or maybe those older TASCOs aren't of the highest quality.

        The bottom line is that you knowing it is a peculiarity of your optic is good enough.

        LR1955

        Comment


        • #5
          "Tasco" explains it I think.

          Comment


          • #6
            Yes, higher magnification means more details and more accuracy. So definitely theres a big difference..

            Comment

            • Klem
              Chieftain
              • Aug 2013
              • 3507

              #7
              My understanding of the parallax adjustment is so the shooter can co-witness the focal plane of the target with the focal plane of the reticle. If your eye is exactly centred in the scope for every shot then parallax is not an issue because the light coming from the target through the reticle into your eye is exactly straight. Keeping your eye dead centre is a difficult ask however so manufacturers allow us to adjust the planes which when coincidental eliminates the distortion of the reticle relative to the target. Your eye can then be off-centre and yet the reticle does not 'move'.

              Scopes designed for close ranges do not have expensive parallax adjustments and it aids speed. Parallax is fixed in the factory at a range where it is likely to cause the least amount of missing the target. It will be about the 130yard mark. Ever noticed the distortion in a fixed parralax scope when looking at a 50yd or even 25yd target? At closer ranges the parallax distortion will have less effect on accuracy than at longer ranges, hence the 130yd bias. Scopes designed for .22's will be parallax fixed around the 50yd mark.

              Your noticing gross movement with the 'head-jiggle-test' at low magnification is because the parallax was way out relative to what you were looking at. The amount of shift is predicated by the diference between the focal planes, not the magnification. At higher magnifications this shift is more obvious.

              Comment

              • BjornF16
                Chieftain
                • Jun 2011
                • 1825

                #8
                Hopefully I'm not stating the obvious (I have no desire to be renamed "MOTO")…

                A specific parallax setting (for those with adjustable parallax) is only good for a particular range (i.e. 100 yds, 200 yds, etc). My local precision marksmanship trainer has his students find parallax setting for each range at which we shoot. He uses the sandbag immobilization method with the "nodding head" to check for parallax. Students record this for each "data shoot" (first 3 lessons).

                You can then select the proper parallax when you engage target at that particular range. Some scopes have sloppy "lash" in the parallax knob and must be adjusted from one direction (Leupold scopes are allegedly notorious for this). My SWFA, NF, and Viper PST don't care which direction the knob is turned from.

                On the other hand, I shot my Grendel bolt at Boomershoot from 350-700 yards with a fixed parallax Viper PST scope (100 yds) and didn't have an issue hitting targets.

                ETA: Most scopes with "ranges" scribed on the parallax knob are nowhere close...
                LIFE member: NRA, TSRA, SAF, GOA
                Defend the Constitution and our 2A Rights!

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