Don’t understand parallax? This should help!

Is parallax real?
Parallax. What the heck is it, and why should you care? Many of the better airgun scopes have parallax adjustments, but nobody explains what parallax is or why it’s so important. Is it real, or just something made up to sell costlier scopes?

Parallax is real and can cause you to miss!
I have tried many times to explain parallax, but the message never gets through. People who understand me already know about it, but those who don’t understand continue to miss out on a lot of potential accuracy.

Simply put, parallax is an optical phenomenon that can make you miss your target, even when your sights are exactly aligned! Don’t believe me? How about this? You know that things under water are not where they appear to be, right? A bowfisherman has to shoot in a different place than where the fish appears to be because the water refracts or bends the light. Parallax is not exactly the same, but it acts in a similar way.

How to see parallax
Rest your rifle so it is steady without you holding it. Now, look through the scope at a target. With the scope reticle resting on a small target, move your head up and down and from side to side and watch the reticle move in relation to the target. If the scope’s parallax knob has been adjusted correctly, the reticle movement will be very little, but you should be able to see some movement at all times. That movement is parallax, and it demonstrates how important your eye placement is relative to the scope. If your eye moves, you will aim for the target in the wrong place – like the fish under water.

The U.S. Army trains its riflemen to cancel parallax
Rifle marksmanship is a skill the Army has always needed. In the Civil War, the Union Army discovered that its soldiers had never received even the basic marksmanship training they needed to shoot a rifle in the field, so they had to quickly correct this deficiency. After the war, the National Rifle Association was created to promote rifle marksmanship in the U.S., but the Army didn’t stop there.

They developed a manual of basic rifle marksmanship that breaks down the principles into steps that can be learned easily. One of the most important principles is how to sight a rifle. Soldiers had to be taught to always place their heads at the same spot on the stock, so the sights would always appear the same to their eye (parallax). The Army calls this head placement a spot-weld, meaning that once you find your spot, you have to be able to return to it every time you hold the rifle, as though your head is spot-welded to the stock.

How important is parallax?
Just last week, I was shooting groups at 50 yards with a .22 rimfire. The rifle’s stock has a low comb that makes cheek placement difficult, so for the first 100 shots I was unable to group any smaller than 5 shots in 2 inches! This was a target rifle with a bull barrel, and it should have been capable of half-inch groups at that distance. Then, I noticed a small amount of parallax through the scope, which instantly told me what was happening.

Because the rifle was a semiauto, I was able to place my head on the stock and hold it in the same place for all 5 shots. Once I did that, the groups shrank to less than an inch, with the best one being smaller than a half-inch. THAT is what parallax can do to you!


Three tight shots on the right; two tight shots
on the left. A five-shot group measuring almost two inches
center-to-center, but every shot was made with the identical sight picture.
The culprit? Parallax – caused when the shooting eye moved between shots.

Here is a common symptom of a parallax problem that shooters often blame on “scope shift.” They shoot three pellets into a tight cluster at 30 yards, then the next two go into another tight cluster an inch away. Parallax causes this, but they want to blame the scope. If the scope has a parallax adjustment, they can’t believe that it still has enough parallax to move their shots around.

Parallax is one of the greatest contributors to inaccuracy with any kind of gun. It confounds most scope users because they can see the sight picture so clearly – yet the picture is really moving as they reposition their head for each shot.

Any questions?

14 Responses to “Don’t understand parallax? This should help!”

  • Anonymous Says:

    This sounds ALOT like the situation I thought was caused by the limbsaver stabelizer I tried on my pcp. So here come the questions:

    1. Is there a chance that this is more pronounced at the extrenmes of a scopes recticle range of adjustment?

    2. If so, I plan on opticly centering my scope. What is the best 30mm ultra high adjustable mount for a talon to allow a reproducable hold point?

    3. I currently use a fixed 1″ ring w/a 4-16x56mm optcal…already high at center. What happens to shorter range sighting when you use an even higher mount?

  • Anonymous Says:

    OOPs one more:

    4. is any of this helpful if it is a scope w/ a.o. already?

    turtle

  • B.B. Pelletier Says:

    Turtle,

    1. Yes!

    2. B-Square AA Ultra-high adjustables in 30mm

    3. At short ranges the pellet goes even lower.

    4. I don’t understand this question.

    B.B.

  • Anonymous Says:

    I’ll look into them…
    the last question was because I’d thought I’d heard A.O. (Adjustable Objective) lenses eliminated the parralax problem. At the range today I did the experiment you suggested and could deliberatly move the crosshairs… alot! So I can tell a more comfortable and repeatable setup is the only way to go.
    thanks
    turtle

  • Anonymous Says:

    a great link I just found to help your readers w/
    this topic:

    http://www.eatel.net/~amptech/elecdisc/parallax.swf

  • Matt Says:

    That link doesn’t work for me. But the following link is to a fantastic site that has interactive animations about air riflery. The demo on parallax is here:

    http://www.arld1.com/parallax.html

    It gives a great visual representation of what B.B. describes.

    Matt

  • Anonymous Says:

    B.B.–so if my scope has an adjustable objective -do I just dial it to the correct range to correct this problem?

  • B.B. Pelletier Says:

    What “problem” are you trying to correct?

    You turn the ajustable objective ring until the target becomes as clear as it can be. Disregard the yardage numbers on the ring.

    I will blog this for you on Monday.

    B.B.

  • Jim Says:

    I was wondering if you would be able to help me with a parallax related problem- I’ve gone through all of the issues that I can think of and still can’t get my new Benjamin Discovery to group.
    I mounted a Centerpoint 4×16 AO and the objective is nice and clear at 25 yards when you dial the AO ring right between 20 and 35. I put a cheekrest from an AR15 on the stock comb, and cheek weld is steady as a rock, (cheek weld wasn’t great due to high rings to clear objective bell). I’m firing from between two sandbags, scope appears clear and true, using quality pellets I’ve tried six different makes & weights of pellets- they made only marginal differences, none of them very good. (seems to like CP’s or RWS Super H a bit better), and still seem to get several small groups at different points on the target. I’m pretty sure that the problem is parallax related because when I dial the scope out from 4X to 16X the accuracy goes from somewhat mediocre to horrendous, as attached pictures show. Seems to go from one lumpy oversize group to 3-4 points of impact. Before I succumbed to the urge to use the gun as a golf club, I did take the scope off and try the Williams sights- the gun shot three pellets into one raggedy hole at the same distance that the photos were taken at.
    I shot the attached groups with the side of my face firmly plastered to the cheekrest and my eyes (mostly) on the target (feeling for the round end to feed the pellets). I have shot quite a bit in the past, (can maintain 1.5 MOA with one of my stock AR’s) have decent breath and trigger control, both eyes open, off of a rest I should be getting much better groups than I am. What part of the equation am I missing?

    Thanks much,

    Jim

  • B.B. Pelletier Says:

    Jim,

    Very thorough testing. It makes my job much easier. The fact that the groups break up with the power change demonstrates there is something wrong with THAT scope.

    I tested the discovery with a Centerpoint 8-32 and got sub half-inch groups at 50 yards, so the gun is capable of that. Most of the groups were around the 0.75″ size. That’s for five shots of JSBs in .22 caliber and Crosman Premier Heavies in .177.

    You have proven that it’s the scope with the open sight test, as well as the low-power test. Stop beating yourself up and get that scope replaced.

    One other thing – are you always shooting from the same fill level? In other words, always testing with a topped-off gun, or starting at 1800 psi or something consistent? The rifle does fluctuate as the pressure drops.

    B.B.

  • Jim Says:

    Thanks for the confirmation on the scope. I took it back where I bought it and swapped it for another one of the same model. Mounted that in same rings, eye relief, etc. Dialed the objective reticle until I can just about read “birchwood casey” on the little target stickers at 25 yds/16 power. The difference between the groups shot at 16 power and that shot at 4 power isn’t as great as on the scope I returned, but I’m getting “batches of small groups”. I did notice that even with my cheek in the same spot, I can press slightly harder or softer on the cheekpad and watch the reticle move in about a 2″ smiley shaped arc on the target.
    My impression is thus that the amount of “uncorrected” parallax on this scope is still pretty high. Is this something that Crosman/Centerpoint might be able to correct? I’m not sure if my expectations of this scope are realistic, but I know that I expect to be able to shoot better groups with a magnified scope than I can with V-notch sights. If I was repeatedly seeing a 1″ or larger group at 25 yards at low and high magnification, I’d just accept that that’s the limit of the gun’s accuracy coupled with my current abilities, but I get “3 in a hole here, 3 in a hole here and 4 all over the place”. Is there anything else that I might be overlooking?
    By the way, when testing I do fill to “about” 1900 PSI, shoot 10, top up to the same pressure and shoot again.
    Thanks again for your advice.

  • M. Scully Says:

    B.B.,

    You wrote:

    "They [the U.S. Army] developed a manual of basic rifle marksmanship that breaks down the principles into steps that can be learned easily."

    What is the name/number of this manual?

    Is it available online? If so, do you have a link?

    Thank you.

  • M. Scully Says:

    Oh yeah… Google.

    It was very late. Don't know why I didn't think of that.

    Thanks!

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