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Apr
14

Parallax… he ain’t your friend

I try to keep my posts as applicable to all forms of shooting as possible. Today’s post isn’t. It’s about Open guns only (but does apply to any optic used on a firearm to a greater or lesser degree). The subject for today, boys and girls, is parallax.

Ok, so, the illustration at the top is not a perfect set of images, but hopefully you get the point. I had to jury rig something to hold the gun steady against a target that would show the parallax in the scope that the iPhone could also see. A bigger challenge than I anticipated! Anyhow, what you see is parallax demonstrated on a C-More scope – as I move the camera to the left in the left frame, the dot also tracks off the target to the left. As I move the camera off to the right in the right frame, the dot tracks off the target to the right. Meanwhile, the gun remains pointed in exactly the same spot. You can see that location in the center frame.

It’s quite popular in the industry to advertise red dot scopes that are “parallax free”. I’m here to tell you, that’s marketing hogwash. It’s essentially impossible to devise an optical system (especially one that relies on a reflected LED emitter) for this intended purpose to be parallax free. The designer can’t control your arm length, the distance to target, nor your eye focus, all things required to design the lens arrangement to prevent parallax. But, low/no parallax is a desirable feature, so all of them claim to have “no parallax”. Instead of just taking their word for it, let’s examine how to tell for ourselves, understand the practical impact of parallax on our shooting, and understand how to account for it.

I’ve basically shown you how to test for parallax in the image at the top of the post, but if you can do it on the range, or with an aiming point that’s more distant than the phone jack in my picture, it’ll make the exercise more clear. Pick out your aiming point, and lock the gun down somehow (vise, sandbags, shooting bag), insuring that the gun will not move while you’re performing the drill. Holding the gun is not a good plan, as the gun will move while you move your head. Once the gun’s locked down, stand behind it about at arm’s length, and move your head back and forth, up and down. Observe how the dot moves in relationship to the target. If it’s extreme, you may have a broken scope, but expect to see at least a very small amount of movement.

As far as parallax goes, the best small red dot scope I’ve owned is an Aimpoint T-1. It still displays a very small amount of parallax, though. Of course, it’s a $600 scope, so you’d expect it to be pretty solid. The C-More shown above is my backup scope, but my primary is very similar. In my experience, they’re both pretty good for C-Mores – I had to get extreme in camera movement to show the parallax in the scope. It’s about 3″ of error at 25 yards. In practice, from a reasonable position behind the gun, you won’t run into seriously negative effects in most conditions. I’ve handled some small red dot scopes that display what I consider to be very unacceptable parallax, though – enough that moving the dot into the left or right quarter of the lens moved the dot off of a 12″ plate at 10 yards. That’s over 30″ of error at 50 yards! Even small errors in centering the dot perfectly at 50 yards with that scope will put you outside the A-zone before you even break a shot! If you scope has that magnitude of error, you need to replace or repair it.

The practical effect of parallax is that your sighting system (the dot) appears to be pointed somewhere other than where the gun is pointed, resulting in your hits appearing in a different place than where you called them when you broke the shot. The error compounds over distance and is magnified the further the dot is from the center of the lens. What this means is that any time you make a precise shot, you need to be certain that the dot is centered in the lens. In fact, you need to change your concept of an appropriate sight picture with a red dot sight – it’s not just “dot on the target”, it’s “dot on target AND centered in the lens”. When you sight the gun in, make sure every shot is fired with the dot centered in the lens. When you shoot groups, make sure every shot is fired with the dot centered in the lens. When you’re shooting at longer ranges, or on tighter shots, make sure every shot is fired with the dot centered in the lens. The worse the parallax on your scope, the more important this becomes. Doing so will insure that all the hard work you’ve done on trigger control will be put to use where you intend it to be! (you’ve been doing work on your trigger control, right?)

Believe it or not… you can have parallax with iron sights, too, but it’s not as prevalent for the individual shooter. Ever notice, though, that a gun that’s sighted in for you will sometimes hit (and group nicely) at a different point of impact for someone else (and you confirm it’s not a trigger control thing)? That could be due to parallax, as well…

So, keep your dot centered and your hits straight!

About the author

DaveRe

USPSA Grand Master, NRA Instructor, http://re-gun.com/about/

Permanent link to this article: http://re-gun.com/2011/04/parallax-he-aint-your-friend/

3 pings

  1. xre says:

    Got Parallax? http://bit.ly/fcdUcg

  2. C-More Care and Feeding – Zero’ing and Adjustment | Re-Gun says:

    [...] cost and form factor are not without some disadvantages, though. The C-More can suffer from parallax, and can have adjustment creep issues. The windage and elevation adjustments are quite coarse and [...]

  3. Optics Parallax | Bangers and Mash says:

    [...] is a great photo from Re-Gun where the author had aimed directly at the phone jack in the center image. When the eye position is [...]

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