Accuracy of ParallexMarks on SF or AO

Discussion in 'Long Range Scopes and Other Optics' started by royinidaho, Feb 14, 2006.

  1. royinidaho

    royinidaho Writers Guild

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    How much does one have to pay to be able to trust 100% the side focus or Adjustable Objective markings?

    That is, no parallex @ 500 yds when SF/AO is set at 500?

    Thanks
     
  2. Jimm

    Jimm Writers Guild

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    I don't think it can be bought at this time Roy /ubbthreads/images/graemlins/crazy.gif
     

  3. Buffalobob

    Buffalobob Writers Guild

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    From the top sticky.

    [ QUOTE ]
    Warning! Snoozer follows!

    Now can we calculate? Oh, goodie! On a short scope, the objective's focal length must be around 0.1 m considering that there is an erector lens in that tube also. The formula for the distances from a lens of the object and the image of that lens is:
    O^-1 + I^-1 = F^-1
    where:
    O = distance from object to lens
    I = distance from image to lens
    F = focal length of lens

    What I'd like to know is how far we'd have to bring the objective lens in if we shift the parallax correction from 50 m to 100 m. Moving the objective lens relative to the scope body makes no essential change in the value of the variable, O. So how far is the image from the lens when the target is at
    50 m? 100 m? 150 m?

    I(50) = [(F^-1)- (O^-1)]^-1 = [(.1^-1)-(50^-1)]^-1 = .1002 m
    I(100) = [(.1^-1)-(100^-1)]^-1 = .1001 m
    I(150) = [(.1^-1)-(150^-1)]^-1 = .10007 m

    We can now see that we're talking very small parallax correction movements here and that furthermore, the corrective movement required for an increment in target distance decreases rapidly as the distance to the target increases.
    So the answer to my question is, if you move the target from a 50 m distance to a 100 m distance, the objective must be moved .1002-.1001= .0001 m to correct the parallax. In Marekin terms, this is .004". That sounds about right to me considering that the graduations on an objective bell are fairly close together and the objective bell's thread is very fine. This also explains
    why it is difficult for the scope manufacturer to put the parallax marks on
    the bell in exactly the right place. All eyes are closed? Have a nice sleep!

    JHBercovitz@lbl.gov (John Bercovitz)



    [/ QUOTE ]



    I am not a machinist, but it appears that having the marks in calibration would be dependent upon getting the thread on the scope tube and on the AO ring to be started at exactly the same spot every time and for the thread cut to have exactly the same tolerances.

    I
     
  4. royinidaho

    royinidaho Writers Guild

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    Hi, Guys,

    The reason for this thread is I didn't want to hijack an other one.

    But this one may be cross threaded /ubbthreads/images/graemlins/crazy.gif

    Here's the other thread. You'll see where I'm coming from:
    link
     
  5. Dave King

    Dave King Well-Known Member

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    The only real use I can find for the numbers on the parallax adjustment is as an indicators that I'm moving the adjustment... if the number changes position I'm moving the knob/ring. I give no thought or concern to the actual value of the number.

    When I adjust parallax I try to always set to one "stop" value and then creap-up on the correct setting by observing the actual reticle movement on the target while constantly moving my eye behind the objective (no contact with the rifle during this check/setting procedure). Once I'm convinced that the parallax is correct (no apparent movement of the reticle on the target) I begin to shoot and at that point there is no need to check the numbers on the parallax setting (I alread know the distance if it's a requirement either via laser or known measured (KD) range values).

    There is a certain amount of slack in the parallax setup (side "focus") and if you ever bother to check the number you'd probably see a significant amount of "error" induced just by how you approach (clockwise vs counter-clockwise)the parallax free setting.

    My 2 cents on the subject.
     
  6. Bart B

    Bart B Well-Known Member

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    I wish everyone in the world who uses rifle scopes would learn and understand that you do not adjust parallax. And those marks on the thing that changes the range the rifle scope is focused at are usually marked in yards or feet or meters. And those marks don't mean parallax; they mean distance or range the scope's front lens is focused at.

    The only time parallax exists is when the aiming eye is off center from the rifle scope's optical axis "AND" the scope is focused at some distance other than what the target is. How much parallax there is or how much you notice it depends on how far the aiming eye is off the scope's axis and how far in front or behind the reticule the target image's focus is. In other words, if the aiming eye is positioned on the scope's axis parallax will never exist regardless of the difference between focus and target distance or range.

    Regarding errors in marking the focus adjustment, consider the following facts. When a batch of lenses are made, each one does not have exactly the same curved surfaces. Which means they do not all have the same focal length. As there's usually two or three individual lens elements cemented together at the front of the scope that comprise the objective lens assembly. And each objective lens assembly for a given scope make and model will therefore not have exactly the same focal length; they'll vary a few tenths of a millimeter or more. As each objective lens assembly is mounted in a lens barrel that's made to the same mechanical specs for a given scope make and model, it will move the same amount front and back for a given amount of focus change that moves the barrel back and forth.

    All of which means that if the focusing mechanics move the objective lens .0010 millimeter when changing focus from 400 to 500 yards but the actual focal length of the objective lens requires it to move only .0009 millimeter then obviously there's an error. How much will this change things? Nobody will ever notice it.

    If you want to see how critical focusing is for a 150 mm lens (typical of most high magnification rifle scopes' objective lens) at 500 yards, look at a camera lens of that same focal length, then notice how far you have to move it to change focus from 400 to 500 yards. The finest camera lenses in the world are not exactly the same, but the mounts they are put it sure are.