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How does sunlight affect your POI?

 
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Old 11-07-2010, 11:57 AM
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How does sunlight affect your POI?

Where I practice shooting,I shoot East to West.I have noticed for quite a while whenever I shoot in the morning,my impacts are 1moa low compared to the evening,when I am facing the sun.I shoot distances from 600 -1200,and it's 1 moa,regardless of the distance,so I am assuming it's the sun messing with the optics.Anyone else here notice a shift in POI from the sun?If so,how do you deal with it?
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Old 11-07-2010, 12:29 PM
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Re: How does sunlight affect your POI?

Most of the time this is caused by the target appearing to have a different dimension than it actually does. Your brain is aiming at a different spot without knowing it. Things look differently in direct sunlight coming from behind you than they do if the sun is at 12 o'clock.

Try to put a small dot in the middle of the target to focus on. This is in the case of shooting steel anyway.

Otherwise, maybe you need to track down an exterior ballistic's reason for your issue. Perhaps there is a measurable environmental variable going on in the morning where you shoot that would cause this. Either way, to find the answer you will have to put in the time. The truth is rarely found in just a couple shooting sessions.
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Old 11-07-2010, 01:38 PM
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Re: How does sunlight affect your POI?

This is a common variable that has to be compensated for just like pressure, temps, etc.....

Various sun angles will cause a shift in where you perceive the target to be. When the sun is low on the horizon behind you versus infront of you or to your left or right or any other combination or when it is seriously overcast, your perception of the target changes. Even though your crosshairs are perceived to be in the middle of the target. Have you ever shot at a target at medium to longrange in a 9Oclock or 3Oclock wind in the heat and mirage? If so, you probably have noted that the target appears to move in the direction of the mirage which is moving in the direction of the wind. You will be aiming at the target and then just before your trigger breaks, the wind lets up for a few seconds. What happens? The target appears to move back to where it started which moves it off of your point of aim. It isnt the target that moves, rather it is your perception of the target. Different sun lighting angles and conditions has a similar effect on target perception. As far as compensating for it, learn what effects differnt angles have on your POI and make field adjustments for it when you 'set up' for a long shot. Personaly, I zero my rifles when it is densely overcast (which isnt hard to do up here) and that becomes my 'baseline' zero. Then I adjust accordingly.
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Last edited by Michael Eichele; 11-07-2010 at 01:41 PM.
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Old 11-07-2010, 07:47 PM
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Location: Texas
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Re: How does sunlight affect your POI?

One possible answer to your question is temperature change effect more than light chage effect. For targets that are shot with iron sights and typical 6 oclock hold on the target, you get a light effect from bright sun making the target appear larger as compare to lower light. you also get an effect from right or left sun making the target appear somewhar eliptical. My experience with hunting with a scope you are aiming to the middle section of a target and not the edges. The middle is still the middle, unless there is mirage present, which is a different subject.

How would I deal with it, I would use my practice and compensate according if I were you.
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Old 11-07-2010, 09:18 PM
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Re: How does sunlight affect your POI?

"Sunlight and it's position relative to the objective lens of the scope will change your point of impact or where the target appears to be"........out of the book Precision Shooting at 1000 yds sure seemed to change POI beyond 700 yds in my experiences. The amount is hard to quantify (even by professionals), and I haven't found any "rules of thumb" for it. It's also not accounted for in ballistic programs at all..........even more troublesome for alot of folks on this site (or others I imagine) who can't shoot to 1K until they come out west.

As already mentioned here, I suppose only strict practice and good record keeping will make us able to effectively deal with that variable. Also as mentioned, Mirage has it's own effects on our perception of "where the target is".

Separate from mirage, the general rule of thumb (among 1K benchrest shooters) is supposedly......"sun up, bullet up" meaning that if it's sunny, we'll shoot higher than if it's cloudy. I believe there is some real truth to that too.

Still trying to figure it out myself, because most of my "precision" or even really long range shooting up until recently has been in circumstances where "sighters" are taken...........not an option for Snipers or even Long Range Hunters.

Last edited by SBruce; 11-08-2010 at 09:42 AM.
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Old 11-08-2010, 07:22 AM
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Re: How does sunlight affect your POI?

Yeah,I am definately experiencing "sun up,bullet up",I notice that regardless of the temperature.I think I will take the advise mentioned above and zero on an overcast day,and compensate on sunny day's.I shoot with a world record holding benchrest shooter,Tom Sarver,and he told me about "sun up,bullet up",and I thought he was pulling my leg.I have now come to realize,there is definate truth to it.Do they make some sort of shader for scopes?I am not talking about the scope extensions you get with your scopes,but something that blocked out the sun ,almost like a pair of sunglasses for your scope.
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Old 11-08-2010, 09:08 AM
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Location: SW Montana
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Re: How does sunlight affect your POI?

I'd like to see some time laps video through a rifle scope locked on a target at a 1000yrds to see how things appeared to move through out the day. I don't know if I'm quite ready to by into this for hunting, I know sun conditions make a difference during target shooting but hunting is a little different, I almost never have mirage or much of any light issues. I'd like to see some numbers, how much MOA are guys seeing their POI change?
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