The Bullshooter

Thursday, March 09, 2006

Parallax in Rifle Scopes

Telescopic sights have always been something of a compromise. Depending on age, quality of optics and how much adjustability it has built in, will decide just how much of a compromise.

I want to talk today about just one aspect of scopes, that being parallax.

Obviously this will not directly relate to any scope with adjustable objective, or other range focus function. Well, maybe just a little...

Your average run-of-the-mill telescopic sight has been focused at a set distance. Sights designed for rimfires rifles generally are set to somewhere around 70 yards, those for center fire rifles somewhere over 100 yards. This means that at that distance, for that particular scope, there will be no parallax error. At any other distance, there will be some parallax error.

Parallax error is something that can be seen when shooting from a fixed point, like sand bags on a bench rest. Set up a target at a known distance, then align the rifle so that the cross hair is pointed at the center of the target. Without moving the rifle, then move your head from side to side, checking to see if there is any movement of the cross hair on the image of the target. When there is no parallax error, the cross hair will remain unmoving on the target until the image disappears. But if the cross hair moves as you move your head, your potential group size has just been magnified by the amount of movement you now see - and it all depends on how you position your head in relation to the scope from shot to shot.

It is common for a center fire rifle to shoot poorly at a 50 yard target for this reason. There may be an inch or two of parallax error, making the unfortunate shooter crazy since it's only logical to expect groups twice the size at 100 yards.

Of course I have a solution to this problem, at least when shooting from a bench, that does not involve buying an expensive new scope with bells and whistles. Feel free to send contributions to the author's beer fund when you see how well it works...

Set up to shoot from your rest as normal. Now, move your head backwards, away from the eyepiece of the scope. You will see that the circular image becomes smaller than the ocular lens. Centralize this image in the ocular lens, making the black band a perfect circle. If you sight each shot like this you will ensure your eye is in exactly the same place, eliminating the parallax error.

If this is a hard kicking center fire rifle, make sure your recoil pad maintains good contact with your shoulder, or it could be a painful experience.

Many target scopes have adjustable objective, that will focus the scope to a particular range. These have yardage settings on the adjusting ring. It is still advisable to use the side-to-side head method to test for parallax error, because in my experience the settings are not often very accurate. For shooting metallic silhouette you may need to make your own graduation marks for each distance. It just takes another variable out of the equation.

If your rifle has adjustable windage scope mounts, it may be advisable to adjust as close to zero as possible using the mount adjustments before moving the scope adjustments. Modern "image moving" scopes bend the image to be viewed in the center, even when the adjustments are at their extremes. You can set the adjustments at or very near their optimum position by winding each adjustment (windage and elevation) the full extent of its movement, counting clicks as you go. Then, bringing them back half that number will give you center. Lower quality variable scopes in particular can move their point of impact when the magnification is adjusted, and this is exaggerated if the scope adjustments are well out of center.

4 Comments:

  • At 24/3/06 5:37 AM, Anonymous Anonymous said…

    paralax is one thing, but how about dot scopes. All my pistols have iron sites, except one an IZH 35m. Should I go Iron sites? or get used to the dot?

     
  • At 24/3/06 4:03 PM, Blogger Warren said…

    This is a really tough question, because until recently I believed that dot scopes, because they have no magnification, were pretty much parallax-free. Judging from a few postings on the Bullseye-L List, it seems that's not the case.

    MAYBE it's a case of quality of optics - do the better dots have less apparent movement? Is there enough movement to matter? If it's less than 1 inch at 50 yards probably not. But if you're seeing that much at 50 feet, that's enough to concern me, anyway.

    I hate to advise going iron sights IF you will be at a disadvantage. I shot a league last year that included some indoor ranges that were pretty unfriendly to open sights - little to no light over the bench.

    But if you shoot where there's adequate lighting, shooting open sights may not be much of a disadvantage, and I believe it teaches more shooting skills. Once you get older, keeping focus on the front sight can be difficult, but that can be helped by the right correction in a shooting lens.

     
  • At 26/3/06 8:51 PM, Blogger Z said…

    "indoor ranges that were pretty unfriendly to open sights - little to no light over the bench"
    don't understand your comment. I prefer to shoot iron sights where there is no special lighting over the bench. Excessive lights over the bench means glare off the sights and shooting glasses. At my club the shooter can control the flourescent over the shooting bay and I always turn mine off.

     
  • At 27/3/06 5:01 AM, Blogger Warren said…

    Yes, excessive lighting will cause glare. NO lighting will give the picture of sights in silhouette; and shooting at a well-lit target will draw the eye to the target, away from the sight picture. Your range is already well lit without the extra fluro over the shooting position.

     

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