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Understanding First and Second Plane Scopes

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bbush View Drop Down
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    Posted: October/14/2020 at 08:58
First, let me see if I understand this optics thing with first and second focal planes.  I understand the difference between first and second planes as far as the crosshairs increase in size with an increase in magnification with the first focal plane while they stay the same with same increase in magnification with the second focal plane.  I think I also understand how a FFP scope can be to estimate distances by using the dots found in most crosshairs (known object if five feet tall...object covers three mil dots...each mil dot is 3.6 inches at a 100 yards...calculate distance).  I also think I understand the SFP plane scope and estimating distance and how it is usually at the highest magnification (at highest magnification...use same procedure above as the FFP scope procedure to calculate distance).  Now to the point where I think I may have a question.  If you are shooting at a shooting range where all distances are known, is there a advantage of a FFP over a SFP scope?  In other words, if i know the distance is 500 yards and you have both scopes zeroed at a 100 yards, is there any kind of advantage between a FFP and SFP scope?  It is my understanding that if your scope is zeroed at a 100 yards and you know that the bullet drops 38 inches at 500 yards, you would simply turn the elevation dial a total of 38 inches in elevation whether you are using a FFP or SFP scope.  Is this not correct?  Second, it doesn't matter whether your scope is in mils or moa, it is the same amount of turn in elevation (once you convert inches to mils for the mil scope 3.6" equals 1 mil).  So is all of this correct?  Now my big question.....is the only advantage of a FFP scope over a SFP scope is that you can estimate the distance of a known size object at any magnification as opposed to only being able to do it at top magnification on a SFP scope?  Also, is there any advantage in hunting between a FFP and a SFP scope if you are using a rangefinder to determine distances?  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: October/14/2020 at 09:36
Technically 1st focal stays the same size regardless of magnification range in relation to the target.  The 2nd gets smaller or larger in relation to the target as you move it up and down. 

Most people use a range finder to get distance.  Using a reticle is very hard because to get a perfect reading you need to the know the exact size of the person/animal/object you are using to measure.  You mess up just a little and you miss.  AT 600 yards it would be impossible to tell if a dude was 6' tall or 5'6" tall.  But it will make a difference in your formulas and ultimately could cause a miss

Another advantage is dialing corrections.  If you shoot and miss with a 1st, you can use the reticle to see how far off you were and adjust to make a good hit the 2nd shot.  If you shoot and your impact is right 1.5 mils as measured by your reticle then you can either dial in 1.5 mils or just use the reticle to make the correction and shoot again.  You can do that on a 2nd, but either you ahve to have the scope set to the mag that is accurate with the reticle or you have to use formulas to make up for the difference.  Which in and of itself can be a problem because the number on the magnification ring does not always equal the actually magnification you are using. 

Hope all that makes sense.  It makes perfect sense in my head.  lol


Edited by supertool73 - October/14/2020 at 09:52
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: October/14/2020 at 14:57
I'll add to that: both mil and MOA are angular measures, so converting to inches is not only a waste of time, but it muddles what is an otherwise simple explanation.

Also, many SFP scopes do not use max power, but all have a spot marked on the mag ring indicating at what power it is accurate.  Be aware that most - if not all - SFP scope makers calibrate one or both ends of their mag ring, none I know of calibrate every numeric power on the ring.

And yes, the biggest advantage to FFP is correcting on the reticle, since a mil is a mil at 100 yards and at 500 yards and 5000 yards, but the inch corresponding to each is not the same.  So converting just adds an unnecessary step, and under stress, that increases the probability of a math error.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: October/14/2020 at 22:39
follow up shots can be made using 2nd FP no matter what magnification it's on. if the shot impacts 2 moa off, by your reticle, you just adjust your hold by 2 moa, using the reticle, not the turrets. of course for this to work you have to remain on the same magnification that you took the original shot.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: October/15/2020 at 00:15
good point
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: October/15/2020 at 10:03
Wait, no, that is not accurate.  Only at one specific power setting is 1 MOA actually 1 MOA.  At half that power, it is 2 MOA, because the apparent target size compared to the reticle is half as large.  Additionally, if the scope manufacturer calibrated a specific power but not the others along the ring, say for my Hendsoldt 3-12, it is accurate at 6X.  Hensoldt did not calibrate 9X or 4X, so taking it as purely linear is not - until verified - accurate.

You can accurately adjust on a mil or MOA reticle ONLY AT THE DESIGNATED POWER.  Everywhere else on the power ring, 1 mil DOES NOT equal 1 mil, nor does 1 MOA equal 1 MOA.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: October/15/2020 at 10:56
I think he is meaning based on the marks on the reticle, if you bullet impacts 2 moa lines regardless of whether or not it is on the calibrated power, at that point if you do not change the magnification and use the reticle as a ruler and just hold the same amount right or left on your reticle that the impact was off it will move your POI the right amount to make the hit.

Whether or not the MOA is correct at that magnification doesn't really matter as long as you don't change it before your next shot. 

Unless both of our thought processes here is totally off. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rancid Coolaid Quote  Post ReplyReply Direct Link To This Post Posted: October/15/2020 at 12:34
Okay, I see that.  My issue was with the 2MOA part, as it isn't actually 2 MOA, but yes, the distance on the scale will be the same between shots, but the scale, at that moment, is neither mil nor MOA, it's just a consistent scale.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote DWilly Quote  Post ReplyReply Direct Link To This Post Posted: October/16/2020 at 08:12
Supertool is spot on with my thoughts and his explanation was much more detailed. i dial which usually gets close, no matter what the power is set at. then adjust with the reticle for follow up shots. the problem with this is if a spotter is calling corrections in true moa or mil, but i don't have that, so it works for me.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Travis299 Quote  Post ReplyReply Direct Link To This Post Posted: October/16/2020 at 17:49
Good luck to ALL during these terrible C-19 times.

Milliradians, (MRAD or Mil) and Minute of Angle, (MOA) Trivia

(Best with a 6 pack of BUD)

 

Formula for circumference of a circle = Diameter, (D) x Pi, (Pi = 3.14159.)

There are 7200” in (D) diameter of a 100 yard circle. (100 x 36” x 2 = 7200”)

Circumference of a circle at 100 yards = 22,619.448”, (7200” x 3.14159 Pi.)

1 MRAD & 1 MOA is an angular measurement at a specific distance or range.

6,283.185 Milliradians (MRAD) = 1 CIRCLE  (Pi 3.14159 x 2 x 1000 = 6,283.185, mathematical.)

6,400 Milliradians = 1 CIRCLE,  (NATO rounded MRAD.)

21,600 Minutes = 1 CIRCLE, (360 degrees x 60 Minutes in 1 degree = 21,600 minutes)

1 MOA = 1.0471975” at 100 yards, 22,619.448”/21,600 Minutes, (1” rounded.) 

1 MRAD = 3.6” at 100 yards, 22,619.448”/6283.185 MRAD, mathematical.  

1 NATO MRAD = 3.534” at 100 yards, 22,619.448/6400 MRAD.

1 MRAD = 10 cm at 100 meters. (2 x PI x 1000 = 6283.185 MRADs in a circle). 100 meters = 10,000 cm x 2 = 20,000 cm in diameter of 100 meter circle.  20K x PI = 62,831.85/6,283.18 MRADs = 10 cm; for  (1 MRAD at 100 meters = 10 centimeters).

Scope settings MOA or MRAD.

It also depends on what your scope's notion of 1 milliradian is, because the NATO milliradian differs from the mathematical definition of a milliradian.

A mathematically accurate radian is that part of a circle where 2 * PI radians is a full circle, and a milliradian, (MRAD), is 1/1000th of a radian, so a full circle is equal to approximately 6283.185 milliradians (2000 * PI).

But the NATO definition of a milliradian for use in ballistics is that a full circle is split into 6400 NATO milliradians.

Anyway, the difference is not very large, with mathematically accurate MRADs, 1 MRAD is 100 cm @ 1000 m, with 1 NATO MRAD, it is 98.2 cm @ 1000 m. For this reason, if your scope uses mathematically accurate MRADs, 1 MRAD is about 3.44 MOAs, but if it uses NATO MRADs, then 1 MRAD is 3.375 MOAs.


 

The distance of the reticle's movement per click is tan(angle) * distance, however, at longer distances this is not exactly equal to the change in point of impact, because of the difference between changes to the line of sight and changes in the bullet's trajectory.

degrees = MOAs / 60
MOAs = degrees * 60

degrees = MRADs / (PI * 1000) * 180
degrees = (MRADs / 6400) * 360 [with NATO MRADs]
MRADs = (degrees / 180) * PI * 1000
MRADs = (degrees / 360) * 6400 [with NATO MRADs]

MOAs = MRADs / (PI * 1000) * 10800
MOAs = (MRADs / 6400) * 21600 [with NATO MRADs]
MRADs = (MOAs / 10800) * PI * 1000
MRADs = (MOAs / 21600) * 6400 [with NATO MRADs]


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