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Parallax

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/21/2013 at 11:42
wryder View Drop Down
Optics GrassHopper
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See various posts here on the subject but none that address it in quite the way I am thinking.

So, I understand what parallax is.  At least I think I do.  That given:

1) What seems hard to believe is that maximum error at 100 or 200 yards is so small.  Here's what I know.  When I am do not have my eye ball lined up with the center of the scope tube diameter on my Mosin, I might not even hit the target paper at 200 yards.  My scope is high, so if I don't think about it, this can occur more frequently than you'd think.  Need a cheek piece.  But, if I am centered I can keep it with 4".  Not great.  Work to be done.  But the point is...seems to vary widely based on my eye ball relative to the center of the scope.

2) So I was thinking I could get a scope with parallax adjustment and eliminate this problem.  The thing is, if max error is under an inch or something...than parallax adjustment isn't going to solve the huge difference in accuracy that I am seeing.  Or, maybe I actually do not understand what parallax is.

So to restate a different way, I was thinking parallax adjustment would eliminate error caused by my eye ball not being centered on the center of the scope tube diameter. If I have this wrong than a) I've lost confidence I know what parallax is and b) I'd ask: is there any type of scope or feature that eliminates the concern about centering...or is it just that you can never ignore having to ensure you are perfectly centered?

Thanks in advance!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/21/2013 at 15:24
Chrome223 View Drop Down
Optics GrassHopper
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Scopes are mechanically focused at some distance.  100y is typical for a centerfire scope, 50y for a rimfire.

The formula for calculating parallax error is EyeOffset*MagnificationPower*ABS(TargetDist-FocusDist)/FocusDist = TargetOffset.

So with 7mm of eye offset, and rimfire scope focused at 50y, shooting at 200y, magnification at 9x, your parallax error is like 1.9" (worst case).   Probably you're closer to 3mm off, and focused at 100y, which gives only 0.5" of error.

So, chances are parallax is not your issue.  If you try to keep your eye centered in the scope, you'll minimize or zero any possible parallax error.

Its probably the Mosin or the ammo, not the scope.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/21/2013 at 17:53
wryder View Drop Down
Optics GrassHopper
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Thanks for the reply!  Hmm.  That's consistent with other things I've read.  At the same time I thought I'd tested with enough rounds to understand experientially what was happening.  I also realize Mosins can be notorious for inconsistency when not set up "the way the individual wants to be."  Ruled that part out because I can be incredibly consistent if I am conscious about all variables.  Maybe I was just getting lucky every 3 shots.

Telling you...something is weird.  Well, I'll have to have it out again and see if maybe I had drank too much the night before.  Or as you say...maybe the ammo is inconsistent.

Tell you what else was weird too.  I have the rifle zero'd at 100 yards.  I swear it was shooting high at 200 instead of low as the ballistics charts suggest.  Maybe my ammo was on speed?

Darn gun is probably a freak!  ...Thanks again...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/21/2013 at 19:37
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Optics Apprentice
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Originally posted by Chrome223 Chrome223 wrote:

The formula for calculating parallax error is EyeOffset*MagnificationPower*ABS(TargetDist-FocusDist)/FocusDist = TargetOffset.


THANK YOU!!  I firgured there would be a straight-ahead formula to figure parallax error, but had never seen it before.

That confirms that parallax on a fixed-focus scope I have was indeed as bad as it looked during a recent shooting session, wherein I could cause the crosshairs to completely span the target both horizontally and vertically as I moved my eye around in the exit pupil.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/21/2013 at 19:44
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Optics Apprentice
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Wait, maybe not...

Confirm for me whether all measurement units must be the same (i.e. everything in yards, including pupil offset), or are some assumed to be inches, others feet or yards, etc.  Please?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/21/2013 at 19:48
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Optics Apprentice
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I wish I could either edit posts, or think before I type.

Nevermind...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 08:40
wryder View Drop Down
Optics GrassHopper
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Actually I was wondering about the units too...as in the examples we were talking about mm for distance off center, but yards in distance to target.  Assuming we at least need to be talking about meters...and so there is probably a little bit of variance in result from that too? 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 11:34
supertool73 View Drop Down
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What scope are you using?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 13:07
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The reason units don't matter is that all but the pupil offset distance divide out of the equation. I don't know what made me second-guess myself with that second post. Whatever units you use for pupil offset are the units of error the equation will yield.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 13:18
Chrome223 View Drop Down
Optics GrassHopper
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So the eye offset input and target offset results need to be the same units (i suggest mm) and the distances have to be the same unit (yards?) but the eye offset and distance units can be different.

Keep in mind that your exit pupil (something like 14mm to 4mm) does mean your maximum eye offset cant be more than half the exit pupil or you cant see the target through the scope anyway.

By experimenting with this formula you should be able to see that parallax is mainly an issue at longer ranes and higher magnification powers.
Post Options Post Options   Thanks (1) Thanks(1)     Back to Top Direct Link To This Post Posted: March/22/2013 at 13:25
Chrome223 View Drop Down
Optics GrassHopper
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Also, shifting yiur head around without touching the rifle and looking for the reticle to move on the target is the sure-fire way to test for parallax error with different head positions. With parallax error, quite simply what you see is what you get.

With your focus set properly at the target distance (i.e 100y for centerfire scopes, or whatever with side-focus or front occular parallax adjustment) moving your eye will not move the reticle at all.

So move your head around and that should tell you if youre at risk for parallax error and how much. While being consistent about centering your eye in the exit pupil will eliminate error. With consisten centering you dont really need an adjustable parallax scope (but for long range and high magnification its desirable)
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 16:00
wryder View Drop Down
Optics GrassHopper
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Ya it seemed to move in the extreme, but again very anxious now to get back out and see what I see.  Maybe my mind (memory) is playing tricks on me.  But, at higher power I can see what seems to be the last 1/4 of the inside of the scope?  So to ensure center I've basically been trying to center the cross hairs on that ring.  Not sure if I'm painting enough of a picture there to make any sense...

But it is to say that...it seems like when I was moving my head around (because I was doing that exactly to test what you are saying), it seems like the crosshairs would move way outside of the target.  Again it was 2 weeks ago now so I'm probably imagining things at this point.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 16:04
wryder View Drop Down
Optics GrassHopper
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Oh that's an interesting point too.  It's a LER Aim.  I don't have it in front of me but similar to this one: http://www. competetion .com

It's probably a foot from my eye when I shoot.  Now mind you...I'm not moving my head 2 feet side to side looking through the scope.  lol.  So I'm sure my side to side movement is within the 7mm mentioned or whatnot.  Well, in fact I guess it guarenteed or I'd not see anything through the scope...

Sorry...just thinking out loud...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 16:08
Skylar McMahon View Drop Down
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Please follow the rules that you agreed to when you joined the forum.
3. Do not advertise other retailers selling the same or similar products as SWFA, this includes posting links to other retailers for products sold by SWFA. SWFA does not provide this forum as a vehicle to redirect customers to our competition.
 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 17:11
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So LER, long eye relief scope?  Probably their 2-7x model.

Don't take this the wrong way, but I would not put much faith in a cheap Chinese made scope.  It being long eye relief I would think could worsen the parallax problems it already has.  Something must be seriously out of whack internally if it has that much of a parallax shift.  At 100 yards parallax should not be much of a change no matter where your head is behind the scope. 

Cheap scopes like that very seldom work as they are intended to work.  I dare bet if put a Leupold scout scope on there it would solve your issues.  
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 20:26
wryder View Drop Down
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Sorry.  Hadn't even thought twice.  Was just trying to ID a scope.

I will read the rules and intend to follow...but can I just put this out there: people know other places sell scopes.  I just meant to answer the guy's question about what scope I had and this is one that popped up in a google search.

Sorry to offend.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2013 at 20:30
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You know what?  I've been on this board about ten minutes.  I do appologize but I can't live with that rule.  I'm going to go find a board that isn't trying to sell me something.  Please delete my account here.  Thanks to the people who were willing to discuss my actual topic with me.  So apprecaite the information...
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