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Zero Shift caused by light conditions - question for W

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Old 03-25-2005, 04:53 PM
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Zero Shift caused by light conditions - question for W


A few days ago, you said this:
Another area of maintaining your zero can be affected by the light and wind. If you zero in full sun-light and then shoot in an overcast condition, (shadow effect) your poi will be ½” low at 100 yards. If the sun is at 9:00, poi will be ¼” right. If it is at 3:00, poi will be ¼” left.

[/ QUOTE ]

Couldn't explain the light theories (in terms of scope use) to myself in common sense terms but:

...over the last 2 days, with dark overcast alternating with bright sunshine, I have absolutely 100% smacked-in-the-face seen the 1/2 minute up/down taking place. [img]/ubbthreads/images/graemlins/ooo.gif[/img]

Grateful if you'd have a squint at this thread:

to see what yout take would have been on the situation.......

have you any other 'not widely known' zero shift tips (ie other than wind , temp, humidity, pressure)that you'd be willing to share?
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Old 03-26-2005, 12:52 AM
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Re: Zero Shift caused by light conditions - question for W

Hello Browndog: Hope all is well with you. Actually, for many years I thought that it was humidity that caused the poi shift, when it was actually cloud cover and sun placement. In a matter of fact, it was David Tubb who brought this to light and explained it to me.

Just a little trivia; humidity only effects the bullet impact approximately 1/10th of 1%, which equates to about 1/100th of an inch at 1000 yards. Unless it is raining, I do not take it into account; except when using Exbal software as it is part of the equation.

By the way, a 5 mph tail wind will keep your bullet down approximately 1/2 moa. Just in case you weren't aware... Many shooters believe a tail wind will cause the bullet to rise. They are mistaken.
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Old 03-26-2005, 02:49 AM
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Re: Zero Shift caused by light conditions - question for W


All well over here thanks. Hope you saw my pic of the ACI mounted over on the Gen Disc section [img]/ubbthreads/images/graemlins/cool.gif[/img]

I am fascinated by this light-caused zero shift. As you saw on the linked thread, my MPI had dropped 2cm (0.79in)when I check zeroed at 100m in Scotland. It then returned to zero when I fired at home. Never explained it to myself, didn't consider light at all! But now light could be the bulk of the answer! I need to play with this.

Some more questions:

I imagine the answer to this one is 'yes', but are you confident enough in the effect that once you've IDed the light condition you dial it as a change?

How 'binary' is the concept; ie is it 'bright = on, overcast = up 0.5moa'.

or do you have 'shades of grey (gray!)'? [literally in this case!] For eg: 'slightly overcast = up 0.25', 'dusk = up 0.75' ?

Is there on IDed effect for sun infront/behind? ie firing towards/away from the rising/setting sun

Apologies for the 'twenty questions' nature of that; but thanks in advance!

[My brain is currently giving me 'does-not-compute' on the tailwind MPI drop though! [img]/ubbthreads/images/graemlins/smile.gif[/img] I'll need to give that one a lot of brain time. What are your thoughts on the effect of headwind?
Is your tailwind concept cumulative? ie would 10mph push it down 1 MOA?]
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Old 03-26-2005, 10:30 AM
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Re: Zero Shift caused by light conditions - question for W

Brown Dog: You will find that if the sun is behind you and less than approximately thirty degrees in the sky, that, it will cause a poi change. As an example, it is 0900, the sun is positioned at about twenty degrees or so at your 5. You range your target and it is at 400 yards. Your data card tells you that you must dial in 8.25 MOA. But because the sun is at its present position, you will now click up .25 minute of angle more; equaling 8.5 MOA. In your situation, with the scope you are utilizing and where you are located on the Earth, you would click up 2 clicks. If the sun were to the left, you would click left one click; and to the right, right one click.

Remember that this equation is caused from refractionary and secondary light as it moves through all the lenses in your scope. (Similar to when you place a stick in a pool of water ~ it appears to bend). Remove the light from the equation, (Night Time or you are using a dedicated night vision device) and you will have another change. Everything has an equation. Once you practice a bit for where you are, you will have those answers immediately.

Wind is a constant that reacts to airplanes, cars, birds and bullets. It is a molecular mass, and when airplanes hit turbulence it is called hitting the CAT. CAT stands for “Clear Air Turbulence.” So it is impossible for it to cause a bullet to rise simply because it is behind it. When the bullet leaves the bore of your barrel, it is arching up. If there is no wind, there is less resistance; but if there is a wind behind you, the bullet will exit the barrel and hit it, and the wind will prevent it from climbing to its full arch. It is the same with a head wind. It must be at or above 5mph to really notice a difference. Precision Shooting Students are taught to disregard head and tail winds. (Old School of thought).
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Old 03-26-2005, 12:00 PM
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Re: Zero Shift caused by light conditions - question for W

"LIGHTS UP,SIGHTS UP"! Lots of info about light and how it affects your point of aim. I just need the time to type it all out for you. In brief, the more light on the target the better you see the target, your hold on the target changes. More later. [img]/ubbthreads/images/graemlins/grin.gif[/img]
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Old 03-26-2005, 01:50 PM
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Re: Zero Shift caused by light conditions - question for W


Thanks. I don't get enough trigger time to note all these effects definitively; but after your bright/overcast heads-up, I suddenly noticed the effect in action.

I'll look for these effects as the situations present themselves.

So far I've got:

Zero in bright light, then fire in overcast, MPI drops 0.5MOA (or, conditions vice-versa , MPI rises 0.5MOA)

Sun low behind, MPI drops 0.25MOA (is the same true of infront?)

and sun left or right , MPI shifts O.25MOA away from the sun.

Is that correct?


Thanks for your input, I'm not a target shooter, but I always understood -in a military context- 'Lights up, sights up' to be a cry that relates to iron battle sights. [and, in that context, as being based on the fact that -in good light- people tend to underestimate target distances]

I understand how, with iron sights light may affect aiming, but....

What we appear to have here, with scopes , is (with a known target distance) actually the opposite:

'Light's down, sights up!' [img]/ubbthreads/images/graemlins/smile.gif[/img]
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Old 03-26-2005, 09:32 PM
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Re: Zero Shift caused by light conditions - question for W


[/ QUOTE ]
Hoooa 3Sixbits and BrownDog!

Here is some other info you may not be familiar with...

When shooting uphill, the target always looks closer then it is. When shooting downhill the target always looks further away.

And yes, Browndog that is correct.
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