True or false? When the light level decreases (like when a cloud passes overhead) your shot will strike below your point of aim. When the light level increases (like when sunshine floods over the range on an otherwise cloudy day) your shot will strike above your point of aim. Does this play into our 100 and 200 yard matches to any real effect? Does anyone care to share a (simplistic) technical explanation of these effects?
Thanks

True with open irons. Not true with Aperture sights Front and Rear.

Boats

Jim_Borton wrote on May 5^th, 2008 at 3:13pm:


Does this change with the type of insert you have in the front??

I use a cross hair or circle or both together.
Gus

I have always been told "light's up, sights up". And that is how the US ARMY MTU teach it.

The explaination is: 
"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."

Craig

Quote:

   I know I'm a couple months late on this, but just read it today. Subsailor is correct in his quote of the USAMTU teachings. It applies to post front sights. When I first started long range BPCR an "Old Timer" was preaching the "Suns Up- Sights Up" mantra, and my aperture front sights just weren't going along with that dogma. Boats is right, once you have a centered sight picture with an aperture front sight, it's centered. What will cause high or low hits are 12 o'clock or 6 o'clock winds. Enter the wind effects charts for right or left hand twist. Useful little devices.
   Brents' call on mirage changes attributed to the sun, I can't grasp. Mirage is nothing more than the effect of reflected heat from the sun. Mirage is a condition used to read wind direction, as well as possible changes in wind direction, again another useful tool to the shooter. If cloud cover or any meteorological event disipates mirage, one uses other indicators such as tree branches or grass, and wind flags. As well as watching where other competitors shots are going. Light affects sight picture, heat (mirage) is a tool to judge wind direction and behavior. Similar, yet different. Aint it funny how things can be so alike yet so divergant?

Brent,
  I agree that mirage can distort the target and sight picture, but fixed range is fixed range. Once sighter shots establish range, that point is moot. Once you have your center established for elevation / range, hold center no matter how the target looks in your sight picture. Provided of course you are using a circle or crosshair front aperture insert, not a post. I have shot BPCR 1000yd matches in excess of 100 degrees, and one match it hit 127 degrees. Now that's mirage!
   I have seen guys (and I used to be one of them) who get "tricked" by the anamoly you speak of, and will "shade" shots a bit high or a bit low. And miss those shots. I used to get so frustrated with myself, as you can imagine.
 

If you have a match rifle and arperture sights sight in and trust them.  They don't change, if you think they do and start cranking the knob compensating for bad shots you can't pay attention to wind and your positon.  

Boats

Well, I cannot say I can entirely agree with you guys.  Apparently, you think mirage is only important to indicate wind.  I have to disagree.  My ego aside, I have seen things that were not caused by a "bad shot", and I can only explain by mirage.   

Your mileage may vary, but I think mirage will do more than indicate wind.

Brent