Ocular focus & shifting POI... any optics experts here?

[Timber338]

I have the new Vortex Razor AMG mounted on a full custom 338 RUM... the rifle and load are dialed and I've put maybe 50 or so rounds down range with the new AMG scope mounted. Last week I shot a sub 3/8 MOA group at 1000 yards so the setup is performing very well including a very good zero that has not changed.

So yesterday I took some time to finally tune in the ocular focus as I noticed that the reticle was not as crisp as I was hoping for. I probably rotated the ocular in two full turns and it really helped. The reticle is now perfectly crisp at any distance now as long as the parallax is adjusted properly for distance.

But... I just shot for a final practice session for my elk hunt next week, cleaned the rifle and shot a final fouling round, and my POI on all of today's shots shifted down 0.6 inches at 100 yards... from an optics perspective, will adjusting the rear ocular for focus cause a POI shift? It only shifted vertically, horizontal was still perfect.

 

[Timber338]

I forgot to mention that I'm running a 25 moa base, which I think is an important detail. Without knowing how to model the optics, I could see how it might be possible to shift POI down by adjusting the rear ocular focus with a canted base...

I'm going to call Vortex up tomorrow morning and ask them... hopefully they can get me in touch with one of their optics guys.

 

[Pdvdh]

It sounds like it did. I have no idea whether theoretical optics knowledge will provide an answer. I wouldn't trust the answer to the extent I'd trust bullet impacts on the target - no matter what.

I would certainly re-zero and then not touch the focus until the hunting is over.

 

[BallisticsGuy]

There's no reason adjusting the focus diopter would change POI. I suspect you changed the performance of the rifle by cleaning it. Have you characterized how your POI intersection travels as you approach copper equilibrium?

FWIW, I never clean a rifle that's grouping well, especially right before I need it.

 

[Rich Coyle]

There's no reason adjusting the focus diopter would change POI. I suspect you changed the performance of the rifle by cleaning it. Have you characterized how your POI intersection travels as you approach copper equilibrium?

FWIW, I never clean a rifle that's grouping well, especially right before I need it.

Same here.

 

[Pdvdh]

There's no reason adjusting the focus diopter would change POI.

Even if you're theoretically correct, that as designed, there should be no affect on POI, are you able to state there will be no affect on POI if the scope isn't 100% properly constructed and operating as designed? What if the scope is defective? Still no reason?

 

[Timber338]

So unfortunately this seems to be one of those mystery problems. The gun and load are dialed tried and true. About a week ago (prior to adjusting the ocular) I shot several groups at long range which varied between the .1 and .3 moa out to 1000 yards.

I talked with Vortex at length which included two different phone calls and two different answers from them about whether or not POI could be changed by rotating the ocular. In the end they went to their engineers and confirmed that theoretically there would not be any POI shift.

As Phorwath pointed out it could happen for example if the scope has a defect.

Vortex suggested it could be a parallax issue. I somewhat doubt that due to the small group size (sub 1/4 moa) that the rifle was shooting before I focused the ocular as well as after. The accuracy of the rifle remains in either scenario. I guess if I held my cheek weld consistent I could still shoot accurate groups to a consistent POI with a parallax issue but at sub 1/4 moa I think that would pretty darn hard to do.

And if the rifle were experiencing some kind of defect to cause POI shift, it seems like it would also negatively impact group size as well. It's hard to imagine a scope defect that only jumps bullet POI between groups... but because of how consistent this rifle has been, the new scope has me concerned.

 

[Pdvdh]

It's hard to imagine a scope defect that only jumps bullet POI between groups... but because of how consistent this rifle has been, the new scope has me concerned.

I understand the concern. What's changed? A new scope.

There's nothing more frustrating than identifying the source of defective scope-caused problems. Especially a scope that dies slowy.

Troubleshooting scopes that progressively fail slowly over time is very similar to diagnosis of some human diseases... difficult to diagnose until the symptoms become bad enough that a Maytag repairman could identify the problem. No scope failure is ever good news, but if I have one fail, I'd like for all the lenses to fall out on the ground at the same time.

Hope there's nothing wrong with yours, because there's not one enjoyable thing involved in the troubleshooting process. Unless you enjoy frustration.

 

[Bravo 4]

It's hard to imagine a scope defect that only jumps bullet POI between groups.

It's hard to imagine, but it has happened to me. I thought I was going crazy, having to rezero the scope constantly but was still shooting little groups. I sent it in and was told it was defective. I couldn't get an answer on what it was, just told it wasn't anything I could have done. They didn't fix it, instead sent me a brand new one (the one they kept was about a year old). The new one was sold instantly.

 

[Bravo 4]

No scope failure is ever good news, but if I have one fail, I'd like for all the lenses to fall out on the ground at the same time.

I agree, at least the problem could be ID'ed instantly.

I had lots of problems with the predecessor to the scope in my previous post. The lenses didn't fall out but cracked with the first rounds down range the first time it was ever mounted.
Cracked is being too nice, it cracked with the first round and split all the way down with the second.

 

[bigngreen]

Depending on how you adjust your paralax changing the clarity of the reticle you could have been of on the paralax which if your position is good would change your poi but shoot excellent.

 

[BallisticsGuy]

Even if you're theoretically correct, that as designed, there should be no affect on POI, are you able to state there will be no affect on POI if the scope isn't 100% properly constructed and operating as designed? What if the scope is defective? Still no reason?

Yes because I know how scopes work and how they're built. Simply stated, ^^^ that's not it. If you're able to elucidate some possible mechanism for this to happen without image distortion then please do.

If it were parallax then you'd almost certainly see shot dispersion inside individual groups, not among them. If his cheek weld were consistent enough inside a group not to have parallax induced dispersion then one would think he'd be consistent between sessions. I could see this might not be the case but I'd personally chalk the parallax angle up to a very squishy maybe with some I doubt it frosting. I could be totally incorrect here but it's just not how I've ever seen it work out.

Air conditions changing along with OP cleaning the gun before his confirmation group is easily enough to explain it. This is exactly why competitive shooters don't generally clean their guns until the groups open up. There are other possible causes but without being able to interact with the system and data log it anything we come up with is pure conjecture. Anything said here so far also ignores another fact of life. Things change. With rifles being used like laser beams they need to be aligned/calibrated/zero'd (call it what you want) before each use especially after having been transported.

Honestly I don't think it's anything to worry about provided you verify POA=POI at the local sighting range one you get to the area you'll hunt. Before every match I shoot we have a sight in period for this exact reason. These are all seasoned competitors and they all pull their rig up to the line and launch a few to make sure everything is on. I will routinely adjust my zero .25MOA or thereabouts from one match to another.

 

[Timber338]

I don't understand the turrets mechanism well enough to know if the turret/lens mechanism could "jump" or "slip" and then hold tight for a while (3-4 shot group). I honestly can't imagine any problem with any part of a rifle that does that. Bedding, action, etc. it is just odd to shoot so accurately and then jump between groups. In my little brain the scope would be the only thing possible if that.

 

[BallisticsGuy]

I don't understand the turrets mechanism well enough to know if the turret/lens mechanism could "jump" or "slip" and then hold tight for a while (3-4 shot group). I honestly can't imagine any problem with any part of a rifle that does that. Bedding, action, etc. it is just odd to shoot so accurately and then jump between groups. In my little brain the scope would be the only thing possible if that.

That's entirely possible. I've had scopes with twitchy erector springs that would only sometimes return clicks as they're dialed in and out. Those were defective. Diopters aren't in the turrets so let's ignore his adjusting of the diopter. What's left?