Parallax Adjustment on Scope

Discussion in 'Long Range Scopes and Other Optics' started by DUKFVR, Jun 4, 2010.

  1. DUKFVR

    DUKFVR Well-Known Member

    Messages:
    49
    Joined:
    Sep 17, 2009
    I am looking at buying one of the Leupold 4.5 X14 X 40 mm Obj.scopes with the B&C reticle. I don't shoot over 300 yrds or haven't yet & really don't plan to unless I upgrade my equipment & skills more . My question is is the adjustable objective really needed ? Any help or comments would be greatly appreciated. THANKS!
     
  2. NomadPilot

    NomadPilot Well-Known Member

    Messages:
    510
    Joined:
    Aug 28, 2009
    What's your target? Hunting at 300 yards or paper? That'll make a difference in the need for absolute precision. For hunting I don't think you need AO at 300.
     

  3. winmag

    winmag Well-Known Member

    Messages:
    2,528
    Joined:
    Dec 23, 2009
    I no longer buy scopes without a paralax adjustment function. My reasoning is simple. When paralax is adjusted properly, your groups shrink considerably. For me this builds confidence in my equipment, and ability as a shooter, and even confidence in my hand loads. All of wich I consider important for streaching the legs on my firearms.
    All that being said, shooting a deer at 300yds and less Ive never taken the time to adjust the paralax for something that big that close.
    Is having a P/A scope a necisity? probably not in this case (imo), but its a very usefull function when you streach the legs of your firearm. Practice at longer ranges builds skill and personal ability. You may be suprised at how well you can shoot when you tip the scales in your favor:)
     
  4. DUKFVR

    DUKFVR Well-Known Member

    Messages:
    49
    Joined:
    Sep 17, 2009
    Sorry about that. Deer & Elk hunting mostly.
     
  5. I wouldn't even consider a non AO scope on a 300 yard gun. I would only buy an adjustable objective.
     
  6. LouBoyd

    LouBoyd Well-Known Member

    Messages:
    770
    Joined:
    Oct 15, 2007
    Lets put some numbers on it . Any non AO scope will be focused at some fixed distance. That distance is chosen by the manufacturer, but is typically 100 to 200 yards, maybe 50 yards for 22 and handgun scopes. How much error can it cause? if your eye is at the edge of the exit pupil the point of aim at the target can apparently shift by:
    Dt Distance to the target from the scope.
    Ds Distance from the scope to it's (factory or not) setting where the target image is focused on the reticle
    Rs radius of the scope's objective lens
    Rt radius of point of aim error at the target.

    Rt = Rs *(Dt-Ds)/Ds

    Note that Rt switches sign indicating which side of the target the error is located depending on whether Dt or Ds is larger.

    Units. Ds and Dt must use the same units but it doesn't matter what units. Typically yards or meters.
    Rs and Rt must use the same units but it doesn't mater what units. Typically inches or millimeters.
    All are linear measurements, not angular measurements. (not mils or moa).

    So if the scope focus distance is 100 yards and you have a 50mm objective scope (radius 25mm or approximately 1 inch) parallax error will be zero at 100 yards. For a target at 50 yards (or 150 yards) the maximum parallax error would be a half inch. At 200 yards it could be 1 inch, at 300 yards two inches. At 1000 yards it could be nine inches.
    If the scope was focused at infinity instead of 100 yards the maximum parallax error at any distance would be only one inch, but it could have 1" of parallax error any distance.

    It's unlikely that you would hold your eye right at the edge of the exit pupil, so the actual parallax error is likely to be half or less what the above equation indicates. This is not related to the quality of the scope or it's magnification. Scopes with larger diameter objective lenses can have proportionally more parallax error.

    I use fixed focus scopes on rifles (mostly AR's) that I shoot offhand at up to a few hundred yards. On any rifle shot from bipod or bags I use adjustable objective scopes. An AO scope which is set incorrectly can introduce more parallax error than one with a fixed objective. I leave my AO scopes set to 2/3 the longest distance I'm likely to shoot that particular rifle. That's keeps the maximum parallax error to well under under an inch at any distance if I don't have time (or forget) to set it.
     
    Last edited: Jun 5, 2010