crosshairs "moving"...does this make sense?

[.22LRGUY]

Hey guys, I had a strange thing happen with a scope today and I'm wondering if you can offer any input. I'm going to describe what I saw in layman's terms, if you can offer any ideas...I'd be very grateful.

So, I set-off to a range today to sight-in my new .17 HMR set-up, scope was a Mueller 8-32x44, range 100 meters. I was shooting from a bench, and was at this range as a guest on a busy Saturday. The range officer who met me there assisted in the sighting in. I had magnification at about 16X and was on paper on the first shot. Within 8 shots, I was within the 1" dot on 2 consecutive shots and presumably, the job was done. Keen to put a few more down range, I dialed-up the magnification to 24X (the upper range at which I read this scope remains clear) and took a shot. Off the 1" circle. Second shot, a flyer. Third shot, miss again. All shots were within a 3" circle, but then I noticed something strange. (sorry, long lead-up)

Looking through the scope, with the rifle supported front and back, if I moved my eye slightly to the left or right, the crosshair seemed to "drift" on and off the target's 1" dot. Should this happen? I can't say I've ever noticed this on any other scope I've owned. I can't say its confidence-inspiring either, as it suggests that every time I look through the scope, I may, or may not be looking in the exact position and thus, not sending the bullet to the center of the crosshairs.

As a non-member of this particular club, I had limited time...so there wasn't an opportunity to change the magnification again or try to understand what was going on.

I'm sure other factors contributed to the so-so results, not the least of which was the heavy trigger and my lack of familiarity with the gun. That stuff aside, what might be causing this issue with the scope? Is it a "phenomenon" with a name?

Thanks for taking a moment to respond,
Rob

 

[zimbu]

What you're describing is parallax. It will become more of an issue the closer you get to the target. Many hunting scopes are not adjustable and are set to be parallax free at 200yd. If your scope is a tactical/target scope it will have a parallax adjustment in the form of a side 'focus' knob or an AO (Adjustable Objective.)

 

[jjohnwm]

It's called parallax, and it exists in all scopes. Objects (targets) at different distances focus at slightly different points within the scope. Only images that focus on the same plane as the crosshairs will have no parallax. Objects at other distances are slightly ahead of or behind the crosshairs, so when you move your head away from the center of the field of view, you are essentially "looking around" the crosshairs at the image and therefore the crosshairs move on the image. It's more observable at higher magnifications.

You can get scopes that have adjustable objective lenses, or a third turret on the side, which allow you to adjust for different target distances. If you don't have this feature, the typical centerfire scope will be parallax-free at 100 or 150 yards. Rimfire scopes are set somewhere around 50 or 75 yards.

Remember also that if you practice good form and keep your eye centered behind the scope eyepiece, you won't experience parallax at any distance. It only appears when you move away from the centerline of the scope...but, especially under field (hunting) conditions, that's easier said than done.

John

edited to add: Zimbu, you beat me to it. There were no replies when I started to answer, but I type slow!

 

[vviking]

It's called parallax.
A scope without an adjustable objective can only be parallax free at the distance set by the manufacturer. Usually 100 - 200 yd's.
All scopes are sensetive to eye alignment, as your eye moves off center a dark crescent will appear at the outside fringes of the F.O.V., and that shot will not go where you thought it would.
Target/long range scopes adjust to eliminate parallax at varying distances, but still need a centered eye to put 'em in there.
You can test by putting your gun, unmoving, on bags and moving your eye about behind the glass, when parallax is eliminated the intersection of the crosshairs will stop moving.
Also, if your scope has A.O., dont assume the markings for various ranges are accurate.


Many 'fliers' have gone wide, blamed on the load or the bedding, when all the while it was me who hadn't learned to shoot.

zimbu, jjohn, Both of you can use more than one finger?.

 

[Lex]

You probably already know this but sometimes when you adjust the elevation or windage a scope doesn't "seat" in the new setting at first unless you tap the adjusted dial once or twice with, say for example, an empty cartridge casing. In my experience this can cause wandering impact points.

I would try this before I attempted any more radical fixes. You have nothing to lose. It certainly has worked for me and has been part of my routine for many years now. Good luck.

 

[M1shooter]

parallax.

 

[Ron AKA]

I presume your scope looks like this?

If so, that knob on the left as you look through the scope is called a Side Focus. It sets the focus to a specific distance from 10 yards (according to Mueller) to infinity. At the same time it should zero the parallax at the same distance. Parallax is the ability to look around the cross hairs by moving your eye up and down or side to side. When parallax is zero, the cross hairs should look like they are drawn right on the target -- no movement when you move your eye. Here is a more thorough explanation of parallax.

One problem you can have is that when the target is in focus, the parallax is not zero, or the other way around. Here is a method to set up your scope so you get zero parallax and perfect focus at the same time. You only need your minimum focus distance to work with so it does not have to be done at the range.

Parallax/Focus Adjustment

Their explanation is somewhat overdone and can be confusing. Simple steps are:

Set up a highly detailed target with very fine text at 11 yards (1 yard past minimum focus). Clamp your gun so it cannot move. Put your magnification at maximum - 32 in your case.

1. Adjust the side focus until the target is in perfect focus.
2. Do the head bop to see if the cross hairs move. If they don't you are done.
3. If not use the eye piece focus adjustment to adjust until there is no parallax.
4. Go back to step 1 and refocus with the Side Focus knob. Repeat this step 1, 2, 3 cycle until you have both perfect focus and zero parallax. Then you are done.

With this style of scope for accurate target shooting you have to adjust your focus and at the same time zero the parallax with the side knob for each distance you shoot at. Generally adjust for focus first, then do the head bob to check parallax is zero, and tweak the same knob if it is not.

Hope that helps some,

 

[.22LRGUY]

Ron AKA~that is the scope, and thanks SO much for the time all of you have taken to help me sort this out. I'd read plenty of great things about this scope, and was reluctant to assume it was a scope issue. That's not to say there aren't better options out there. I should have also mentioned, in my original post, that the "layman's terms" thing had more to do with me..it was the only way I could describe it. I fully assumed that posting it in the Optics forum would generate some much more informed discussion...which it has, so thank you all.

Having read your responses, I'd like to offer a little more about what I did (in my own ham-fisted way, and see what you think.

The range was 100 meters, and I "found focus" at approx 90 yards on side focus. I focused looking through the scope, not looking at knob's calibration. Here's the curious thing, when the range officer asked if he could take a couple of shots, he looked through the scope, adjusted the focus to distance way closer and put a couple right on target. (I was watching through a large spotting scope that didn't looked focused to my eye) I got behind the rifle again, and the target down range looked as blurry as the image through his spotting scope. Guess I didn't make the connection about the focus as I thought in both cases, he was compensating for some vision issue he had. Maybe he was, but maybe he saw the parallax issue I didn't.

Going to do some reading this afternoon, and try to get a better grasp of this parallax thing. I've certainly read the term plenty of times, but admit that I've never really understood what it meant. Going to try and correct that today.

Thanks again guys, and please don't hesitate to add anything here that might help. As I only get the chance to shoot once a week or so, it may be a week or more before I can update with my findings. Please feel free to drop-in and see how I'm making out!
Rob

 

[Ron AKA]

The range was 100 meters, and I "found focus" at approx 90 yards on side focus. I focused looking through the scope, not looking at knob's calibration. Here's the curious thing, when the range officer asked if he could take a couple of shots, he looked through the scope, adjusted the focus to distance way closer and put a couple right on target. (I was watching through a large spotting scope that didn't looked focused to my eye) I got behind the rifle again, and the target down range looked as blurry as the image through his spotting scope. Guess I didn't make the connection about the focus as I thought in both cases, he was compensating for some vision issue he had. Maybe he was, but maybe he saw the parallax issue I didn't. Rob

Yes, two possibilities. The Range Officer may have ignored focus and adjusted only for parallax. You should have seen him bopping his head around if that was the case. Or, he could have been adjusting for his particular vision.

In any cause if YOU cannot set the focus right (and you did it right -- without looking at the SF knob) and have zero parallax at the same time then you need to adjust the scope as I suggested above. It can make a very big difference. Also keep in mind that the method I suggested is an unconventional one, and you will have some tell you it is all wrong. It is not the method you are likely to find in your scope manual. But, I know of very experienced benchrest shooters who use the method. I have used it last with a $700 Sightron scope and can produce targets like the one below at 100 meters. My experience is that it really does work. Also, don't worry about what the distance scale says. Many scopes are quite inaccurate. If you want it right, as a last step, put some tape over it, and mark it up with the correct distances.

 

[.22LRGUY]

Thanks for that Ron. My CZ is off for a little trigger work, when I get it back, I'll work on making the adjustments to the scope and report back.

I'll dig-up this thread then, and update it.

Thanks again,
Rob