"Light's up, Sights up".... diffraction?

A

alinwa

oft dis'd member
Could you guys give me a tutorial on the effect of lighting changes at 600yds and beyond?

I've read posts here before that mention but don't quantify the effect. Experienced shooters all seem to have a way to handle sun/cloud changes...... experienced I AIN'T! I got bit today and haven't got a handle on "why".

I'm not much concerned with theories as to how it works, just WHAT TO DO when it's sun/cloud/sun. In other words, if I start my run under cloud cover and the sun pops out, other than stopping how do I change my hold.

any help?

al
 
Did a search on it myself and found... "That happens when the light intensity goes from low to high and the shooter's eye iris gets smaller. When that happens, the aiming bull appears larger and the front sight post will still appear the same size, but when touching the bottom of the apparently larger bullseye, the shots will go low. In darker (dimmer?) light, the eye's iris gets larger and the bullseye appears smaller causing the front sight post to be closer to the center of the bullseye; the shots go higher. The `light's up; sight's up' rule of thumb is very correct for post front sights whether they are hooded or not. Typical correction varies from a quarter to almost a full minute elevation change depending on the person's eyes and light condition changes as well as atmospheric conditions. When aperture front sights are used, this doesn't apply; the bullseye centers in the round aperture regardless of how big it appears, but the aperture size might need to be changed to allow a better sight picture."

Sounded pretty good to me :)
 
lngrng said:
Hmmmmmmmmmm.....
I always heard it "sun up, bullet up, sun down, bullet down".

Anyone else know?
Click to expand...
you are right sun up,bullet up,sun down, bullet down.
thermals from heat coming off the ground cause the bullet to rise,also mirage changes cause light rays to bend before getting to your eyes.this makes the target appear higher than it actually is,this is why shots go high in a boiling mirage.shooting in mirage and changing light conditions can make you or break you.practice in the worst conditions you can with someone spoting your shots.In compition shoot in the best conditions you can get that day or read um and hold.thats what seperates the men from the boys.:D:eek:
 
Al,

Where I first heard the 'lights up/sights up. lights down/sights down' was with shooting NRA High Power with a Service Rifle i.e. post front sight. The common explanation at the time was that as the light gets brighter (sunny day) the black bullseye (all we can really see with the naked eye) gets a bit 'washed' and the lighter colored 'white' (buff) area gets brighter - making the bull appear slightly smaller. Correspondingly, when the sun goes behind a cloud or on an overcast day, the black seems darker and 'bigger'. Where this affects the aiming is with a six-o'clock hold (pumpkin on a post) as the apparent 'edge' of the bull changes location slightly. The amount is fairly small - 1/2-3/4 moa typically, but enough that the mantra took hold. Some shooters use a center-mass hold in an effort to counter or at least minimize the effect. In theory (assuming this theory is correct) this shouldn't really affect people shooting with aperature sights (as Keith mentioned) or with a scope, as any one 'edge' of the bull or aiming point doesn't really matter any more.

A few years ago I noticed it still seemed to bite me even when shooting with match rifle (aperature, or circle-inside-a-circle-inside-a-circle) sights, or even with a scope (Any Rifle/Any Sight). Not as much, but still *some*, and definitely repeatable. I asked around a bit and never really got a solid answer other than some people noticed it, some didn't. A few theories about differences in mirage off the ground displacing the target image optically, etc. but nothing really accepted as firmly the 'lights up/sights up' mantra. Usually someone cites examples of leaving a scope zeroed on a target from early in the morning til later in the day, checking periodically and seeing the crosshairs moving all over the place (I'd really, really, REALLY like to see a condensed video clip of that!) so the mirage theory may have some merit. Others debunk it by claiming the observed movement is from heating of the tube (or the gun if its mounted on one). Add into that some ranges get weird thermal updrafts and down drafts and it gets hard to separate the theory from the real world effects.

In the end... shooting and observing the effects in the real world are probably the best solution. Taking notes definitely helps, but keep in mind what applies at one range might not at another. Some days every shot is a sighter...

YMMV,

Monte
 
Monte,

This has been my experience too..... conflicting information.

OR JUST MY CONFUSION!!! (((You gotta' realize here that I'm one of those guys who can't remember "rules".... I have my own way of visualizing how to move my poi in a scope too. For me, I must visualize the movement of the whole system, ie, "when you move the scope THIS way, the barrel moves THIS way" etc...)))

I've heard Marks' (ingrng's) way and also the "lights up, sights up" way and while they may be saying the same thing I CAN'T REMEMBER! Or I just can't decode the rules.....

Here's my deal.

This last match I had some sighters that just went WHACKY, like 6"-8" vertical for no apparent reason in my mind. It screwed my headgame to the point that I just slammed my shots downrange. I still did extremely well, my rifle is like a laser and did its job despite my efforts to out think it. But I'd like to have a plan. I handled the wind OK and the rifle Q'd out the vertical but my sighters were scary. I got messed up enough that I thought my gun was throwing shots so I tried to clean it at the wrong time and caused myself some unnecessary grief. As usual, this match was a great learning experience and exhausting, it's just that in replaying my shots I've got this vertical problem that the others (better) shooters attributed to light changes.

Problem was, it seemed that in my case the sunny shots went UP as ingrng predicts......maybe "lights up, sights up" plays out in reverse? Like "raise your REAR sight when the light goes up???" Like drop the muzzle, just raise the rear?? not raise the sight in the rear??

In any case, I'd like to know what folks think of this ONLY AS IT APPLIES TO SCOPES.........not lighting conditions on the irons.

thanks for anything........


al


BTW, ingrng, we missed you man! It was a tense race there for a while....
 
Well Al, I can't help you much more with the lighting portion. I admit I need to keep better notes myself on things like that - or at least right them down where I can find 'em again later ;)

Just thinking things through out loud here, so I can understand what you saw...

You're saying you started your unlimited sighter period in sunlight, had some clouds come over during your time, but finished up your sighter period back in the light, and shot your record shots in bright light. Is that about correct?

I have to say... 6-8", or 1-1.25 moa seems like an awful lot for 'just' lighting. I could however see it happening for either wind or mirage. Did the wind shift direction at all during that same time, and come back to its previous direction. Up drafts and down drafts are some evil, evil things. The other option... perhaps the clouds or a stray breeze 'killed' your mirage. I have seen that happen before at a 600yd range in western Washington. Sitting there watching (and cussing) the mirage boiling up and slightly to the right off an asphalt roof from below the line of fire, as I could barely see the scoring rings at all. Fought it for the better part of the day, and thought I was getting some wonky vertical as I'd get some 7 o'clock 9's - til I watched a very light breeze come in, 'wipe' off the boiling mirage for about 10 seconds and the target *moved* from where it was to where I was holding off at. Very educational about the optical displacement of the target due to mirage. Normally I think we mentally factor this in to our wind corrections, but this particular day, it was almost entirely a force of its own.

Without seeing exactly what you saw on that range, its hard to really say what went on. I'd say maybe get ahold of Cody and see if the local HP shooters (who have been shooting there far longer than the BR crowd, and get fairly consistent feedback with every shot being marked) have noticed any little 'tricks' peculiar to that range like the ones I mentioned above.

We need to get you to a LR match @ Douglas Ridge there in Portland... that range will drive you abso-friggin-lutely batty ;)
 
great points ,one thing i might add

in my experience with mirage is if the sun hits the target the hold for mirage is generally doubled.i use the flicker as a gauge on the amount of hold if the sun hits the target that flicker will increase in length and not to mention increases proportionatley with the wind .if the flicker disappears then expect verticle because the wind just shifted towards or away from you that is not the time to shoot ,at least in the 1000yd game. the easiest way i can say it is that i will always aim to the upwind end of the flicker .we get a ton of mirage and it is ussually the only reliable wind indicator. hope this helps sir. tim in tx
 
Monte,

I don't think the highpower guys can really see it..... although George and Peter both talk about the extreme vertical on this range. It DOES show great windblown vertical due to some berms in the range but this stuff was "different" IMO. I just popped some unexplained (to ME) sighters. My groups were OK. The BR shooters talked as if it were normal light/dark vertical dispersion.

On my own range I normally only shoot 100-350yds and have never noted a vertical "lighting dispersion" but I shoot a lot of sighters. Of course you can't do that during the shots for record at 600yds.

al
 
tim in tx said:
in my experience with mirage is if the sun hits the target the hold for mirage is generally doubled.i use the flicker as a gauge on the amount of hold if the sun hits the target that flicker will increase in length and not to mention increases proportionatley with the wind .if the flicker disappears then expect verticle because the wind just shifted towards or away from you that is not the time to shoot ,at least in the 1000yd game. the easiest way i can say it is that i will always aim to the upwind end of the flicker .we get a ton of mirage and it is ussually the only reliable wind indicator. hope this helps sir. tim in tx
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ahh-HAHH!! You may be onto something there my frien'! And this backs up what Monte said.

I distinctly remember one really high shot. The target cleared up to crystal clear, I could see bullet holes..... I figured I'd shoot the spotter off and BAMM! the shot went wayyyy high.

scared me.

al
 
alinwa said:
Could you guys give me a tutorial on the effect of lighting changes at 600yds and beyond?

I've read posts here before that mention but don't quantify the effect. Experienced shooters all seem to have a way to handle sun/cloud changes...... experienced I AIN'T! I got bit today and haven't got a handle on "why".

I'm not much concerned with theories as to how it works, just WHAT TO DO when it's sun/cloud/sun. In other words, if I start my run under cloud cover and the sun pops out, other than stopping how do I change my hold.

any help?

al
Click to expand...



Back when I was switching from HP to LRHP and Benchrest I became very interested in this topic. Unfortunately most of what I found and read was “over my head” since it was more scientific than shooting related. The human eye and light refraction is every bit as deep if not more than the study of ballistics. But, I will try to share some things that might help and even I understand (anyone will get it).

One thing that has helped me quite a bit is to remember that you really don’t see anything “as it is” since what you see is either the reflection or refraction of light from that object. Since you are seeing a refraction (unless you are shooting at a mirror) then anything that changes that light can cause the brain to interpret the size, shape, color or location somewhat differently (although usually very small amounts). It is far too complex of an issue to just narrow it down to the iris, there are far too many chemicals and parts to your eye that work in conjunction with each other (see
http://www.howstuffworks.com/eye2.htm when you have some spare time) not to mention that each human brain may interpret the refraction of light off of an object differently.

Let’s look at mirage. Cold air is more dense than warm air and refracts light at a different speed than warm air. When the temperture change is enough in a tight layer of warm/cold air then we have that thing we all know and love…mirage. Because in most cases for us shooters the warm layer is on the bottom thus causing the upward bending of the light refracting from the target which a lot of shooters report that it makes the shots go high (sun up bullet up). I know we are not specifically talking about mirage but it is directly related. Think about it, if a small change in a layer of air causes light to bend then I submit to you that things like light intesity, light position and yes even the humidy level all change how and where we see the target! If you have a hard time getting your head around this then think about looking a fish in the water, the water causes the light to bend and the fish is not where you see him and if you try to gig him at night you know that the position of your light will change the position of where you see the fish (not that I would ever illegally gig a fish when I was a kid). :eek: Here is an good piece on mirage that will also help http://en.wikipedia.org/wiki/Mirage when you have spare time.

So, back to the real question. What do you do? Let’s see, I’m half way through my group, number 2 wind flag is shaking it’s tail like a dancer and number 3 is pointing in a totally different direction, the earth is rotating and moving my target 1.7 “ while my bullet is in flight, the spin of my bullet is moving it to the right 7.2”, sun pops out bright from behind a cloud, mirage rears it ugly wave of spite, my round is starting to cook in my chamber, do I adjust that front rest or squeeze that rear bag a little harder, how’s my cant, what is the wind meter doing, did I zero my scale between the 11th and 12th round last night, light is bending before it reaches my eye, that onion on my burger at lunch has decided to become a volcanic erruption, Newton was born to bring me pain…if you can sort through all that and more in an instant with constant changes to the variables then I humbly lay my rifle at your feet. In my opinion it is a mental game at that point and although the light does have some influence it is the least of my worries! I shoot both scope and aperture and what works for me is to focus on the few big things and shoot through the rest! That 8” verticle wouldn’t have screwed my headgame but my headgame would have screwed me with an 8” verticle. :D

Al, I have never seen a light change of any kind that would make an 8” difference at 600 yards but I hope some of the above helps you find an answer.

Tony
 
al sir,mirage is funny sometimes it can really hurt you or it can really help you.

what you might do to get an idea of how much the light can do on a bright white target would be to shoot a cardboard target and then jack another round in a pop a white target right next to the dark target and notice where the impact is ,a stark difference in contrast will change the poi .how much? the main problem is how much to hold for your particular eyes and scope combo,at ranges like hawks ridge sometimes i will be hitting clay birds at 1000 in the dirt and be dead on and when i go to the bright white paper and i am a foot or two out but have a real pretty group , so my scores sucked and so i had to change up my thinking, this method i use presently has helped my score agg come up ,and kept all this years bullets on paper as well.i am sure some of the hawks ridge guys could elaborate on this as well they have had it down for many years. good luck mr. al sir, tim in tx
 
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tim in tx said:
what you might do to get an idea of how much the light can do on a bright white target would be to shoot a cardboard target and then jack another round in a pop a white target right next to the dark target and notice where the impact is ,a stark difference in contrast will change the poi .how much? the main problem is how much to hold for your particular eyes and scope combo,at ranges like hawks ridge sometimes i will be hitting clay birds at 1000 in the dirt and be dead on and when i go to the bright white paper and i am a foot or two out but have a real pretty group , so my scores sucked and so i had to change up my thinking, this method i use presently has helped my score agg come up ,and kept all this years bullets on paper as well.i am sure some of the hawks ridge guys could elaborate on this as well they have had it down for many years. good luck mr. al sir, tim in tx
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Well, yahhh...... that's how it was :confused:

I was (the gun was) shooting 2" groups but I couldn't tell where they'd land!

I'da' made a real showing but shot #15 never showed up on the paper! A 4-shot group really kills the agg eh.....Ohhh Well, live and learn. :)

al
 
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