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Estimating Distance W/MIL DOT Fixed vs Variable

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Ranger007 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ranger007 Quote  Post ReplyReply Direct Link To This Post Topic: Estimating Distance W/MIL DOT Fixed vs Variable
    Posted: February/03/2011 at 12:32
I am a novice shooter and tried to find the answer and so far no one can give a logical answer that makes any sense. So this you could say is a last resort and depending how this goes could be a first resort from here on out.
 
First let me explain some issues that have lead me to ask this question.
 
I am running a REM 700 SPS short action chamber in .308 WIN. with a SS 16x42 Fixed MIL DOT. Yes I know that the optic is a little much for the rifle but so far it does exactly what I want it to do.
 
My question is and please forgive me if a fudge the correct terms etc. If I am using the optic referenced above to estimate distance to the target how does a variable power scope affect range estimation? Meaning if I estimate my range to the target to be 150 yards in a X3 magnification and I then zoom in to the target on say 10 power the MIL DOT recticle does not change? Is that correct? If it is correct and the rectilce does not change how does one know what power to be at in order to estimate range estimation?
 
I am asking this question becuse the people at MIL DOT Master can not answer this. 
 
Any info/assistance would be greatly appreciated. 
 
Ranger007
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SVT_Tactical View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SVT_Tactical Quote  Post ReplyReply Direct Link To This Post Posted: February/03/2011 at 12:49
Variables with SFP will have a certain power in which the mildots are true, normally posted in an owners manual or somewhere.  However FFP scopes are true on all powers.
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Urimaginaryfrnd View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Urimaginaryfrnd Quote  Post ReplyReply Direct Link To This Post Posted: February/03/2011 at 13:21
Most scopes use the highest power as the correct power to mil dot rangefind which makes sense  because that allows you to more accurately estimate tenths of a mil.  Some manufacturers use a specific power as the correct one to mil at like 12x which Bushnell seems to like.  Recently we have been seeing some first focal plane scopes which will mil rangefind at any power and as you zoom from low power to high power the reticle grows larger in appearance but the dots are always one mil apart.  The traditional style second focal plane scopes the reticle always appears to stay the same size.

"Always do the right thing, just because it is the right thing to do".
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Ranger007 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ranger007 Quote  Post ReplyReply Direct Link To This Post Posted: February/03/2011 at 13:44
Thank you very much Urimaginaryfrnd and  SVT_Tactical!!!!!!!!
I am tracking. Yippee
Ranger007
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jonoMT Quote  Post ReplyReply Direct Link To This Post Posted: February/03/2011 at 15:37
To expand on that, let's look at three examples: 1) Your 16x fixed-power scope, 2) a variable FFP scope and 3) a variable SFP scope. In all three cases, we'll range a target 12" in size @ ~150 yards.

  1. The fixed power scope is essentially like an FFP scope, just with one power. So the ranging equation is straightforward and always the same. 12" = ~.333 yards. Multiply that times 1000 and you'll get the distance it would be if the target spanned 1 mil. Now, say the target actually spanned 2.2 mils (Let's be realistic. It's hard to get a precise number looking through a reticle). Divide 333 by 2.2 and you get 151.36 yards.

  2. With the FFP scope, the equation is going to be exactly the same as with the fixed scope. The only difference is that when you change magnification, both the reticle and the target change size - together. With an FFP scope, the target will still span 2.2 mils whether @ 3X or 15X. So there is no additional math or having to ensure that you're on the scope's ranging power. You just have to be on a high-enough power to accurately measure.

  3. An SFP scope will also require no additional math either - but only at its ranging power. Consider a scope, such as a 2.5-10X with a ranging power @ 10X. Measure 2.2 mils across that 12" target @ 10X and the math is the same. However, what if you want to range @ 5X? You would look through the reticle and see that the target only spans 1.1 mils. If you divide 333 by 1.1 you will get 302.73 yards. So either you need to first multiply the mils by the ranging power divided by the current power, e.g. 1.1 x (10/5) before dividing. Or divide the results by the ranging power divided by the current power, e.g. 302.73/(10/5) = 151.36

    Take the scope down to 2.5X and your same target now subtends only .55 mils. You would either need to multiply that times 4  (using 10/2.5) or divide the resulting distance by 4 (606.45 /4) = 151.36.
So you can see that ranging with an SFP scope is most easily and best done on its ranging power. Or at worst, on half that power so you can just multiply or divide by 2, depending where you prefer to factor that in. For a scope that only goes up to 10X, SFP is not much of a limitation. Personally, I never range anything under 300 yards and for more than that, you really want 10X. For a scope with a ranging power of 22X, IMO SFP is a serious limitation. My preference for a scope that goes beyond 10X is definitely to go FFP. Aside from not worrying about being on a specific ranging power, you can also use the reticle to correct your point of aim at any power.
Reaction time is a factor...
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Ranger007 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ranger007 Quote  Post ReplyReply Direct Link To This Post Posted: February/04/2011 at 21:15
Excellent. I am tracking and I actualky understand it too. Thank you all very much greatly appreciated! Ho-Ah!!
Ranger007
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