scope power

Discussion in 'Technical Questions & Information' started by jbeam, Jul 1, 2012.

  1. jbeam

    jbeam New Member

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    This is a stupid question. But i noticed one day when i was shooting my 223 that if i set the scope at a diffrent power i would hit diffrent. Now nothing drastic. but if i would go from say 9 power to 16 at the same range. my bullet impact would change. Had a guy tell me the same thing the other day. I was just wondering if someone could tell me why this is. Maybe im just messed up, i don't no. Any information would be great thanks.
  2. cycloneman

    cycloneman Well-Known Member

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    maybe your seeing the target better

    cheap scope

    is the scope moving when you turn the power ring? bad rings?
  3. JLA

    JLA Well-Known Member

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    Your shots will be closer to POA with a higher magnification value because you can see the POA better.

    Less magnification puts you closer to 'iron sights' accuracy.
  4. Hammerslagger

    Hammerslagger New Member

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    Optical systems found in sights, telescopes, and binoculars are compromises.

    Inexpensive optical products often (must) have lots of undesirable compromises because high quality costs "big money" to both design and manufacture. Big money optics have few compromises, and often cost more than the firearm that they are mounted on.

    You do not have to buy the very best or highest priced firearm optical sight to get a good quality product. However, cheap variable power scopes are usually of all around inferior quality to inexpensive fixed power scopes. If you have a cheap variable power scope, you best bet is to set it near its lowest power and leave it there.

    P. S.-- To directly answer the question of post #1. The phenomenon you refer to is usually caused by something known parallax in the optical system. High end optical sights eliminate or greatly reduce parallax. In expensive variable power scope sights usually have lots of it. Using higher magnification usually aggravates parallax greatly in low price point scopes.

    When the Redfield "3 to 9 power Rangefinder" (mounted on a Remington 700 with a heavy barrel) was near STATE OF THE ART for sniping in the late 1960's, operators were advised to determine the range to target using the variable power rangefinder feature; and THEN DIAL DOWN TO 3X TO MINIMIZE PARALLAX, AND SHOOT THE TARGET.
    Last edited: Jul 1, 2012
  5. LDBennett

    LDBennett Well-Known Member

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    I guess the whole concept of parallax is hard for people to understand. But I'll try to explain it here. The difference in point of impact for changes in magnification may indeed be caused by parallax.

    Any scope has NO parallax for one particular target distance. Unless the scope has an adjustable objective lens system (distance focus for the front lens element, sometimes located on the side as well) the parallax can not be removed no matter the cost of the optical system. Parallax is related to the optical system not being in focus for the target distance. The effect is most noticeable with higher power and most scopes of power greater than about 10x have adjustable objective. But the user must adjust the scope for the target distance.

    Parallax exits because the target image is not focusing on the cross hairs but in front of or behind them. The rear lens system (ocular lens system) should be adjusted independent of a target so that it is focused on the cross hairs. When both the objective and the ocular lens system are both in perfect focus there is no parallax error. There is no parallax error at any distance IF your eye is perfectly centered in the scope optical system. The parallax error is only apparent when your eye is viewing off the optical center of the scope.

    Typically 3x to 9x scopes do not have adjustable objective (front) lenses but are pre-focus at about 150 yds whereas 22 scopes w/o adjustable focus objectives are focused much closer, like at 50 yds. Scope of less than 10 power probably do not need an adjustable objective especially if the user keeps his eye centered in the scope view.

    The trick to getting the scope adjusted perfectly is to view the sky through the scope and adjusted the ocular lens system (rear focus adjustment) until the crosshairs are sharp and contrasty. Only then should you attempt to adjust the objective (front) lens system. You can just dial in the distance using the scale provided or adjust the target image for maximum sharpness. There will be no parallax at that one distance and moving the eye back and forth off the optical center of the scope will verify that the cross hairs do not move on the target with the gun held steady. This is a good way to verify the scope is in focus at the target image distance.

    Hope this helps.

    As an aside, cheap scopes include poor optical systems where it is possible that the whole of the systems optical center could be significantly off. Good lenses assembled off center makes for a problem scope. Better scopes have both better glass and more accurate assembly. In general, I have found that any scope that is much cheaper than $200 list price is suspect for both poor optical performance and poor mechanical assembly. Both cause problems for scopes. I have all kinds of scopes from Leopold and Burris to Bushnell. I have found for my uses (mostly scopes above 10X) that the Bushnell Trophy scopes are about the least expensive scope that I can buy that has both good optical performance and mechanical assembly. Cheaper scopes just do not seem to cut it for me.

    LDBennett
  6. Hammerslagger

    Hammerslagger New Member

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    Nice, dertailed post #5, LDBennet.

    I will add that most inexpensive scopes are not constructed as internally robust as higher priced scopes. For example Bushnell's 4200 series scopes are advertised to be designed and able to take 10,000 shots on a .375 H&H Magnum rifle; while their 3200 series is just 2000 shots on the same gun.

    Another item that bears mention is the power of any optical system. More power with diminished brightness and clarity is typical of inexpensive variable power scopes, both sighting and spotting. Real world usable power is limited by both the diameter of the objective (front) lens group and the diameter of the body tube on a scope sight. There are some high end and high priced exceptions to the following; but, generally speaking, variable power scopes are limited roughly as follows: 20 mm x 3/4" tube, 7X max.; 40 mm x 1" tube, 15X max.; 50 mm x 1" tube, 20X max; 56 - 60 mm needs bigger than a 1"tube and is limited to about 30X max.

    Generally speaking, when you try to exceed the above power limits, image quality and brightness goes down greatly, and you end up seeing less not more.
  7. LDBennett

    LDBennett Well-Known Member

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    The real limit for the amount of brightness you see is the human eye. The rule of thumb is the objective diameter should be 5X the power to get full brightness. The iris of the human eye is normally limited to about 5 to 7mm. Scopes (and binoculars) with more than that 5x relationship between the power and the objective diameter are a waste as the eye limits the brightness.

    While there are 7/8 inch scope tubes as well as one inch, and 30mm, Leopold makes a tactical 34mm tubed scope and my Millet for our 50BMG is 35mm with special rings. The manufacturers of scopes have to do ray traces through a potential scope design to determine the tube diameter that will work. It is a function of both the magnification, the length of the optical system, and the objective diameter.

    Another limitation that the objective diameter places on the user was recently brought home to me (again) when I used a gun with a small objective diameter. That limitation was the diameter of the field of view. That is, with this particular scope your eye had to be perfectly lined up or you suffered a blackout. The alignment of eye to the scope was very critical and an increase in the diameter of the objective would have made that effect less.


    LDBennett
  8. Jim K

    Jim K New Member

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    There is a pretty good discussion of parallax in sights at:

    http://en.wikipedia.org/wiki/Parallax#Parallax_in_optical_sights

    Basically, parallax comes into play when the shooter does not keep his eye in the exact same position relative to the scope from shot to shot. This is easily seen if the scope is put in a fixed position and the reticle put on a fixed point. If the person lookiing throught the scope moves his head from side to side, the reticle appears to move across the field of view.

    Parallax is due to the relative position of the reticle and the lenses inside the scope; it can be reduced or eliminated for a given range, and more expensive scopes have adjustments to do that. But manufacturers of less expensive scopes design a scope based on an assumption of what it will be used for. For example, a four power scope generally used for deer hunting, will usually be parallax free at 100 yards.

    But other factors can cause the proplem to OP mentions. Adjusting scope power means moving or turning a lens or lenses. If the lens is not nearly perfect, it can introduce the kind of error the OP is experiencing, which has nothing to do with parallax.

    Jim
    Last edited: Jul 2, 2012
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