Dr. Scopelove, or How I Learned to Stop Worrying and Love Parallax


(From the undiscovered, secret “Art of the Rifle” archive vault of shooting past. I thought I would take a break from the number crunching of scope testing and write about something quick and easy. Apparently I have a mental disorder that compels me to collect and crunch numbers.)


Parallax seems to be in the mind of more shooters as they are starting to notice that a lot of scopes have side focus knobs. There’s a knob there that says “Parallax” with yard numbers and such, so it has to be a big deal. I must have noticed that knob at some point, as evidenced by this old article on my blog.

Before I entered the abnormal mental state of coming up with numbers to crunch in relation to shot group stats, I had a different abnormal mental state of coming up with creative shooter-related diagnoses for what turned out to be a gun that needed some accuracy work. One of the things I got hung up on was parallax, as my scope on that rifle, formerly the FN PBR-XP, currently the “Mark Deux”, is not equipped with a side focus knob. Sometimes not having the means to adjust something can give one the idea that that lack of that ability is a serious and significant deficiency.

Earlier in the year I was loaned a sample of the upgraded SWFA 3-15×42. Perhaps upgraded is the wrong word, because I don’t know that it’s actually intended to replace the 3-9×42, but it would just make sense. As Nigel might say, “This one goes to fifteen. Well that’s six more, innit?” I thought it would make sense to compare them and see how that turned out.

The newer 3-15×42 has a side focus knob. One of the things I wanted to compare was whether that resulted in better capability for precision. Before I did that, I needed to establish a baseline with my tried and true 3-9×42. What follows is my submission of evidence that I actually made my journey to the dark land of parallax and returned to tell the tale…

Plan A: “No plan survives first contact with the enemy.”

Armed with the On Target TDS program and some of the proprietary targets that go with it, I set out to attempt to induce parallax error at 100 yards. Those of you who are smarter than me (only the 7,000,000,000 who have that distinction) may have already spotted the flaw.

I have to reverse engineer my grand plans to some degree, because this happened a couple months ago, but the plan seems to have been to shoot 3-4 sessions of 12 shot experiments. Each target consisted of three columns of four bulls. For those actually keeping track that’s one shot per bull. Since I had three columns, for the left column I would put my eye as far left in the scope’s eyebox as I could while still being able to see the target. For the center column I carefully aligned my eye in the center. For the right column I moved my eye to the far right. I shot them from left to right, which would mean that each group was formed in a “round robin” fashion to reduce the possibility that something would make one group better than the others due to some outside condition. After a few outings I should have collected large enough sample sizes to be able to tell something.

After shooting the 12 shots I compiled the results into three groups of four shots, one for each column, using On Target TDS. On target allows for precise measurement of group size in extreme spread and mean radius, the location of the exact group center, and the measurement of the distance of the actual group center from the point of aim in terms of horizontal, vertical, and total deviation.

The raw targets look like this:


Here are the composite groups:


Parallax Left


Parallax Center


Parallax Right


Parallax Sum

After analyzing my first target I remember being confused. In retrospect I should not have been thinking anything after only four shots on target, but I did not see what I had expected to see, which was three distinct points of impact that correlated directly to the directional change in eye position. Nothing in life ever turns out that perfect.

Later in the day I tried again with exactly the same format. This time I felt like I shot a little better. Note that in this case the group centers were all very similar. There are differences in group size, but I felt that could be easily attributable to having more difficulty with sight picture and eyestrain at the edge of the eyebox.


PM Parallax Left


PM Parallax Center


PM Parallax Right


PM Total Parallax

You might have noticed little to no shift in the point of impact in the last three shot groups and the composite total.  I noticed that too.  I now had a total of 24 rounds telling me something different than I expected. I started considering something that I should have thought of before shooting. I emailed SWFA and asked them what the parallax was set to in the scope. Skylar answered the same day with the following: “The SS 3-9 has a fixed parallax set at 100 yards. “ Trying to induce parallax error at the precise distance that the scope is set to be parallax free is not a really good idea for those of you that are sane and well-adjusted.

Plan B: Trying to induce parallax at a distance other than the factory setting.

After hearing from Skylar, I set about researching the problem a little more. I found something interesting on the internet here: http://www.longrangehunting.com/forums/f18/parallax-adjustment-scope-57372/ (scroll down to post #6 by LouBoyd).

Using his equation, RT = RS(Dt-Ds)/Ds, I figured out that at 200 yards the possible parallax error was equal to the radius of my objective lens, or approximately 0.827”. I used 200 yards because of my overdue shooting goal, which is to be able to hit a 4” target within that distance under a wide variety of conditions (understated). 0.827” isn’t much, but I decided to test at 200 anyway.

I placed three targets at 200 and shot them in round robin fashion, with 6 shots each. Here you go:


200 Parallax Left


200 Parallax Center


200 Parallax Right;

Here is a composite target of all 18 shots:

Total Parallax 200

Here is a chart of the deviation, in MOA, of each group.

Parallax Chart

Here is a chart of the mean radius if each group, also expressed in MOA:

Mean Radius Parallax Tests

After seeing that the group that should have been pretty good was not, and that the groups that should have not looked as good, but did not, and after not seeing any particular shift in point of impact, and, after seeing that my groups at 200 yards in which I tried to induce parallax were better than the groups I fired at the distance at which my scope is parallax free… I decided the following:

While parallax is a real thing, the importance of it isn’t universal to all shooters. For long range it’s going to be more important. For my purposes, there are more important things to work on, like follow through, which is what I think happened with the “center eye” target at 200. Once again, it comes down to fundamentals.

18 thoughts on “Dr. Scopelove, or How I Learned to Stop Worrying and Love Parallax

  1. Parallax is only important when you are shooting at a target at a substantially different distance than what the parallax is set to. The SS 3-9×42 is set somewhere between 100 and 150 yards, so in practical terms you will not see any significant parallax issues until you are at very close distances (less than 40 yards) or at extended distances (beyond 400 yards).

    Oh, and I need to think about it, but I am not at all convinced that the formula you used is accurate.


    • But Ilya, I got it from the Internet so all the information is vetted, right?

      I do realize that shooting small targets at long distance will change things. I didn’t want to give anyone the impression that parallax is a hoax propogated by “big optics” or some such thing. BUT, it is easy to get wrapped up in details that are insignificant for a given application, or even a large percentage of shooters who will never fire a shot beyond 300 yards. What was useful for me was figuring out that it’s back a ways on the list of worries, given my current shooting needs.

        • The formula, if I am understanding this correctly (and it has been a few years since I have given any thought to this) assumes a scope on unity magnification and ignores eye relief and eye pupil size. Actual parralax error is a function of eye relief, magnification, objective lens size and distance to target. I’ll think about it a little and make an Excel calculator. I’ll e-mail it to you.

          • Some of us are interested, too! It would be nice to see an answer from someone I’m convinced actually knows what he’s talking about.

  2. Adjustable objective AO or parallax adjustable scopes are very popular with field target air rifle competitors. These guys use the side focus as a rangefinder. Knowing the range is much more important with pellets since velocity is low, and drag is high compared to bullets. Because AO scopes sell well it might seem that AO is a must have feature, but as your experiment showed it usually is not. I often wonder if a 2000 dollar rifle scope is really that much better than a 200 dollar scope. I suppose it depends on how you use it. For the shooter that zeros the scope and leaves it alone I suspect the 1800 dollar difference is wasted.

    • I didn’t know that about the pellet shooters. I hadn’t even heard of field target air rifle competition. Sounds interesting.

      I find that the yardage marker on the knob is not quite consistent with the proper adjustment.

      Having recently used a bunch of spendy scopes, I could see that for niche applications it would be worth it. I’m planning on selling some rifles to fund some optics, because I would rather enhance the capability I can use rather than having a larger collection of things I can’t use as well. It also depends on how much abuse you expect it to take.

      I think that for something tried and true, like a 3-9, there are probably some low cost options that are pretty well done. Ilya’s site probably could point one in the right direction.

    • I can just feel the enthusiasm!!!

      Someday soon I’ll write stuff about shooting again.

      I had fun on the 4th by the way. Good shoot. I wish I could have stayed longer. Thanks for putting that on.

    • You’re welcome. I never really expected anyone to benefit from these forays into the dark world of numbers and measurement of mine. It sure makes blogging more complicated when I have to consult a five tab excel spreadsheet to read my results, and when it’s not easy for me to figure out when I’m the one that made it. Not to mention that what should be a simple 500 word article needs 15 photos of targets and composite targets. It’s probably going to get worse before it gets better, but I have a feeling that one day it will just be shooting again.

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