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#6904323 - 09/30/17 05:16 PM Light refraction, causing a POI shifts and bullet trajectory differences
ChadTRG42 Online   happy
THF Celebrity

Registered: 09/16/09
Posts: 13306
Loc: Lewisville, TX
I have battled this issue multiple times, and have never figured this out. This video was put together by Scott with Vortex Optics. He set up a spotting scope, rigged it up in a box so it would not move at all. Set up a camera to take pics every minute of the day. It shows how your POI (point of impact) can change simply from the lighting and light angle. I have seen POI zero shift problems at 100 yards on several ranges I have shot at. I know I will need to correct and account for it. But to this day, I have not figured out HOW to account for it. I've shot rifles that shoot the same hole at 100 yards, and from morning to afternoon, have a POI shift at 100 yards. I have always liked a cloudy day for shooting and zeroing, just for this reason. It helps give you a more true trajectory path, without having to worry about the lighting and POI shifts. I thought this video was pretty cool! Here's what he said about it:

Scott w/ Vortex Optics
"Over the years I have noticed funny discrepancies in my elevation on different ranges. Particularly at my home range. Even with accounting for Environmental I could have a .4 mil elevation shift at 1000 yards in less than an hour. I think we all have been at a match before and have had that one target that everybody is hitting(or missing) low or high on. Plenty of people have theories of what that is whether it's crosswind jump, atmospherics,lighting,aerodynamic jump,etc..... I still don't know exactly what causes it but what I wanted to do was catch it on film. So I set up a spotting scope with a time-lapse camera attached to it and filmed an f-class Target 1000 yards away for 9 hours(8am-5pm). I attached the spotting scope directly to the judges stand and in cased the spotting scope with a box with just a loophole for the scope to see out of. this way there would be no thermal impact on the scope or wind blowing it ,Etc...it was totally protected from the elements as I wanted to minimize any skewed outcomes. I filmed this numerous times from numerous different platforms, tripods ,judge's stand, 4 x 4 and had a similar outcome every time."



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#6904359 - 09/30/17 05:51 PM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
dee Online   content
THF Trophy Hunter

Registered: 04/10/10
Posts: 5448
Loc: Red River way
I've noticed it being a factor depending on which direction you are shooting too. My home range faces darn near due north and it's rarely a factor there but has been noticed at matches with zero lines facing east or west.
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#6904376 - 09/30/17 06:05 PM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
Kevin1 Online   content
Tracker

Registered: 04/06/11
Posts: 669
Loc: Allen Tx
Mirage is known to cause POI shift.

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#6904457 - 09/30/17 07:43 PM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
RiverRider Offline
THF Trophy Hunter

Registered: 11/11/07
Posts: 8222
Loc: Wise Co.
That's pretty weird. It would be interesting to set that experiment up long term and track humidity, temperature, and other atmospheric data over an extended period while shooting the time lapse.
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#6904476 - 09/30/17 08:13 PM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
Toxarch Offline
Pro Tracker

Registered: 03/09/11
Posts: 1929
Loc: Parker County
"I attached the spotting scope directly to the judges stand..."

What is the judge's stand made of? Perhaps the materials that the stand is made of is expanding/contracting. I'd like to see it left in that same position and another photo taken the next day at 8am to see if it moved back to center. IMO, something is heating up during the day to cause that much movement... unless there's an active geological movement right there.
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#6904667 - 10/01/17 07:09 AM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
P_102 Offline
Pro Tracker

Registered: 09/23/08
Posts: 1125
Loc: Grapevine
This is why the 5 Dot Drill should be done first....I'm pretty sure none of us missed on our own and Buzz just got lucky with the conditions!

peep

P_102
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#6904764 - 10/01/17 10:01 AM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
FiremanJG Online   content
THF Celebrity

Registered: 12/16/08
Posts: 25656
Loc: Wolfe City, TX
I've seen zero shifts, when changing location, and always attributed it to temp and DA. This has merit, I too would like to see a longer term test.
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#6904809 - 10/01/17 10:48 AM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
P_102 Offline
Pro Tracker

Registered: 09/23/08
Posts: 1125
Loc: Grapevine
Enough tests might end up showing a repeatable pattern. That would be pretty cool!
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#6904815 - 10/01/17 10:56 AM Re: Light refraction, causing a POI shifts and bullet trajectory differences [Re: ChadTRG42]
Cleric Offline
Veteran Tracker

Registered: 03/11/12
Posts: 3359
You would have to test muiltple variables and come up with a good measurement system...

Some variables cardinal direction, weather conditions, scope, etc

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