Light- and it's effects on POI

[BENNYBOOBOO]

Hey all- was letting my mind wander last night (wife said I looked stoned out of my mind) and thinking about something I read somewhere that said something about lighting-conditions and possible effect on accuracy.

So for those of us that haven't the foggiest idea where to start- How do lighting conditions effect the relationship between POA/POI? What about direct sunlight (let's pretend that mirage doesn't exist, because that's a whole other topic) versus high-cloud cover? Angle of the sun? How do we measure the possible effects of light before we take a shot (example:Wind=MPH; Light=????)?

Can somebody please give this wandering-mind some direction?

EH

 

[Gary Morgan]

I know from my own experience and from hearing Tony Boyer talk that in a benchrest match the point of impact will get higher as the day goes on. We're not talking a lot, but you can look at targets shot in the morning and compare them to those shot later in the day, and the POI is higher by say a half inch on targets shot at 100 or 200 yards.

 

[Michael Eichele]

Over the years I have always noticed that my POI would change day to day most of the time but not always. The changes were ussually concistent. My POI was either high and left or low and right if it wasnt dead on.

A couple of years ago I had developed a load for my 300 RUM and zeroed it 300 yards. I went to the range a couple times aweek for a month or so. Everytime I went it was cloudy. I noticed my POI never changed. I thought that was strange that every time I went to the range my POI didnt move. Then one day I went and it was sunny. Guess what? It changed. it was low and right. I still didnt make the connection. I rezeroed my rifle and the next time I went to the range I was high left. Still didnt get it.

Then one day I went with my 308. I shot a group at 300 yards and it was dead on. It was very cloudy. With in seconds, the clouds parted and became VERY sunny. The sun was shining bright on my target. Guess what, the next group was low right. It became clear to me at that point why my POI was changing. Now I document if I sighted in during very bright sunny conditions or cloudy conditions. Typically I sight in during cloudy conditions because we have so many gloomy days up here. Then I know if I am setting up on a shot in bright daylight I readjust my zero a 1/4 MOA up and 1/2 MOA left. In other words when I sight in for cloudy and it becomes bright, my POI changes to low right. .25x.5 MOA.

It is more complicated than that for EXACT POI changes. It matters where the sun is. But the bulk of the changes will happen between gloomy and bright.

 

[eddybo]

Have you noticed any increase in the POI shift with increased humidity?

Over the years I have always noticed that my POI would change day to day most of the time but not always. The changes were ussually concistent. My POI was either high and left or low and right if it wasnt dead on.

A couple of years ago I had developed a load for my 300 RUM and zeroed it 300 yards. I went to the range a couple times aweek for a month or so. Everytime I went it was cloudy. I noticed my POI never changed. I thought that was strange that every time I went to the range my POI didnt move. Then one day I went and it was sunny. Guess what? It changed. it was low and right. I still didnt make the connection. I rezeroed my rifle and the next time I went to the range I was high left. Still didnt get it.

Then one day I went with my 308. I shot a group at 300 yards and it was dead on. It was very cloudy. With in seconds, the clouds parted and became VERY sunny. The sun was shining bright on my target. Guess what, the next group was low right. It became clear to me at that point why my POI was changing. Now I document if I sighted in during very bright sunny conditions or cloudy conditions. Typically I sight in during cloudy conditions because we have so many gloomy days up here. Then I know if I am setting up on a shot in bright daylight I readjust my zero a 1/4 MOA up and 1/2 MOA left. In other words when I sight in for cloudy and it becomes bright, my POI changes to low right. .25x.5 MOA.

It is more complicated than that for EXACT POI changes. It matters where the sun is. But the bulk of the changes will happen between gloomy and bright.

 

[Michael Eichele]

Have you noticed any increase in the POI shift with increased humidity?

Never at my 300 yard zero. Only at 800-1000 yards. Humidity has a very minimal effect on exterior ballistics. So little I dont even compensate for it under 800 yards. In my 308's I have seen less than 5" of difference between near 0% and 90+% at 1K and NO left or right changes only vertical.

 

[eddybo]

I am just wondering whether increased water vapor in the air changes the amount that the light is bent making the POI shift further or whether the shifts you are seeing are a constant.


I used to own a cheap spoting scope that had a reticle in it. If you sat it up in the morning pointed at a target by mid day it would move somewhat. I always attributed it to basically the same effect as mirage. I just thought water vapor in the air was bending the light making the target appear to be where it wasnt.

Is this the same thing yall are discussing? Is water vapor in the air bending the light the cause or am I mistaken in assumption? I do not know the answers, just assumed that was the cause.

 

[Michael Eichele]

I dont know about vapor in the air bending light, but I do know that light in and of itself causes the human eye to perceive targets differently for different conditions. This is why we have changes in POI. It is REALLY bad in archery. Not as bad with scoped rifles.

 

[eddybo]

I hear a little bit about this from guys using peep sights and thought that what they were discussing was caused by the lights interplay with the sight and not a distortion in the line of sight. So maybe we are discussing two different aspects.

 

[BENNYBOOBOO]

Wow guys, have gotten a lot of really great responses and possibly a couple new questions.

Maybe I'll just have to start taking better range data-notes. I know I prefer to shoot on a high-overcast day because the target is usually very visible and yet there is minimal mirage to shoot through (it can be a blessing and a curse). However I'll keep better records to see whether or not my POI changes between bright, sunny days and overcast days.

Sun angles and shadows could be important too, but that'll have to be included in notes as well.

EH

 

[Michael Eichele]

I hear a little bit about this from guys using peep sights and thought that what they were discussing was caused by the lights interplay with the sight and not a distortion in the line of sight. So maybe we are discussing two different aspects.

To a point yes. But also no.

Lighting changes in and of itself will change your image percpective for peep shooters and scope shooters alike.

A peep shooter can be worse off than a scope shooter though. If the sun off to one side or the other the sun creates glare on the peep and really adds to the problem. Most shooters will find that their POI will follow the sun throughout the day. When it is to the left, they will hit left. As the day goes on and it gets straight up, the POI will be centered and so on.

We are not talking about huge changes here but 1/2 MOA is very common.

 

[arthurj]

All of these comments are correct. And let me say, I don't have a lot of long range experience.

What I do know for a fact:
Mirage flowing in a certain direction, or boiling, can make your target position appear to change.
The angle of the sun also can alter the position of your target in your scope. When light is entering the scope lens at different angles, the target will appear to be in different positions.
My observations have been that mirage will move the target in the direction it is flowing.
As far as sun position, say for instance you are shooting at a target due north. Sun is rising to your right in the east, when I fire a shot POI tends to shift to the right.

What I don't know:
I don't know the exact physics behind this, or how to quantify the shift of POI.

I am very interested in the physics behind this though. I wish we had a physics professor around...