Calculating calibration of mildot reticle at different magnifications

[coyyote]

Howdy, new member here with a question for the optical physicists of the forum. I am trying to determine how to calculate the distance between mildots of a SFP scope when using a magnification other than the ranging magnification specified by the manufacturer.

To be more specific, the NF NXS 8-32x56 SFP MilDot ranges at 22X. I have been told that if I go to 11X then the distance between the mildots of the reticle are no longer 1 mil but 2 mils. My question which was directed first to NF tech support garnered this answer for what is the inter-dot distance for 32X. "Divide 32X into 22X (the ranging mag) = 0.687. Since this number is in MOA divide it into 3.44 to convert to mils."

Well I was not sure so I did this using 22X divided into 22 X and got an answer of 0.30 mils between mildots. Of course the correct answer is 1 mil! So am not sure of this formula. Others tell me since halving the ranging magnification to 11 doubles the inter-dot distance then doubling it to 44X (which the scope obviously wont do) gives 0.5 mils. Am suspicious of this logic also.

Anyway what I am trying to determine is: A. what is the inter-dot mil distance at 32 X. B. what magnification produces an inter-dot distance of 0.5 milradians.

Appreciate any help. Thanks in advance, and I promise the next query I post in the future will be easier.

Coyyote

 

[Sniper338]

This is why I have FFP scopes lol. Mind boggling trying to figure that out.

 

[coyyote]

Yeah FFP simplifies the issue Am becoming a bit of a troglodyte and have returned to SFP scopes and ditched the "Christmas Tree" reticles for simple mildot for shooting steel out to 1600. Kind of enjoying the simplicity and although still ramping up on the new reticle do not seem to be missing the tech. (Scopes are cheaper too).

 

[mikecr]

"Divide 32X into 22X (the ranging mag) = 0.687. Since this number is in MOA divide it into 3.44 to convert to mils."

The number is not in anything, it's just a ratio.

So at 22x, the ratio is 1:1 of what you're looking at, or dialing(moa or mils).

 

[coyyote]

So if this is a simple 1:1 ratio then the distance between dots at 32X is 32 divided into 22 = 0.69. And 0.5 mils would be 44X, if the scope could get there. Once I get the scope mounted will use a "Barber Pole" and check the 32 X which should be around 2/3 of a mil (2/3 of 3.6 inches) at 100 yards.

 

[mikecr]

Does your NXS dial in mils, or moa?
Is it part number C354 (.1 mil dialing)?
Trying to understand why a connection to MOA was attempted.

 

[hogpatrol]

A: 5"
B: 27.5x

 

[Ned Ludd]

You did everything right except for the last step. The mildot reticle subtensions are not in MOA, they are in mils. So your 22X/22X = 1 mil calculation is correct, as is your 22X/32X = 0.6875 mil. To obtain an interdot arc estimate of 0.5 mil, you'd need to dial up to 44X (i.e. object doubles in size with respect to the reticle interdot arc (distance)>>>means double the mag from 22X). Halving the mag to 11X would increase the interdot arc (distance) to 2.0 mil.

It might be easier to think about this as though you were looking through the scope at a ruler edge. What will happen to the apparent reticle interdot arc (distance) as the ruler image changes in size? The reticle remains a constant size to your eye...the object (the ruler) does not. As the ruler image increases in size (increasing mag), the reticle interdot arc (distance) decreases proportionally. As the ruler decreases in size (decreasing mag) the reticle interdot arc (distance) increases proportionally.

3.44 MOA/mil is a correction factor that is only applied after you first determine the mag-dependent change in reticle subtension in units of mils, which you did correctly. So for the 1 mil (22X/22X) calculation, the corresponding value in MOA would be 3.44 MOA (1.0 mil x 3.44 MOA/mil = 3.44 MOA); for the 0.6875 mil (22X/32X) calculation, the corresponding value would be 2.37 MOA (0.6875 mil x 3.44 MOA/mil).

This illustrates a problem with scopes that have different units for the reticle subtensions and turret values: if your turrets are in MOA and the reticle in mils, you have to make these kinds of calculations in order estimate a required turret adjustment based on the reticle subtensions. The units aren't the same, which means a PITA trying to do the math in your head every time...it's too easy to make mistakes that way. Regardless of whether the reticle subtensions are in MOA or mil, if the reticle units match the turret units, no conversion is necessary...the adjustment you see in the reticle is what you dial on the turret. If you are in this position, at a minimum I would suggest very generous rounding, for example, use 3.5 instead of 3.44. It's rough, but if you have 0.25 MOA turrets, it's easier to convert the turret adjustment estimate using 3.5 (i.e. 14 clicks on a 0.25 MOA turret). For many applications, that estimate will be close enough to hit the target, but not nearly as challenging to figure out in your head on the fly.

 

[SSL]

You could also buy the Strelok Pro app for a fast reference tool. Assuming your reticle is listed in the data base (likely since it is huge) you have an instant visual of which dot to use for a specific distance at any magnification your scope is changed to. Great tool for SFP variable power scopes.

 

[coyyote]

This is a C345 with mil turrets. Sorry I did not specify. I will look at Strelock Pro as that seems like an interesting idea. Thanks for the help guys, the C345 arrives tomorrow to be mounted on my 300 WM.

 

[rbduffer]

Hey Coyyote, a lot of good info here.
I'm an old guy who doesn't like electronic gizmos that end in the word 'Pro'.
And since your scope is mil/mil, the math can be super simplified.

I set up my scope for 'round' numbers that are easy to multiply, then make them fit the magnification power. For example, if I had your scope, I would have this chart on my stock or hashed on cloth tape directly on the mag ring.

3/4 mil = 29.3 mag
1 mil = 22.
2 mil = 11.
2 1/2 mil = 8.8 mag

You don't need to know every magnification and it's much easier and faster. And if your off 2 tenths of magnification, no big deal. Or you can make these marks on the cloth tape mag ring to be exact. My 2 cents.

 

[coyyote]

rb That will work great. Thanks.

 

[mikecr]

NF doesn't show a C345

 

[coyyote]

You are correct. I transposed numbers. C 354, last one on the list. NXS 8-32x56 milradian with Mil-Dot reticle. Illum & Zero Stop.

Friends have already informed me I am going "Back to the Future" with SFP and mil dot reticle replacing an FFP ATACR Mil-R. Next couple of weeks ought to clarify if this was a sound decision.

 

[cowboybart]

You did everything right except for the last step. The mildot reticle subtensions are not in MOA, they are in mils. So your 22X/22X = 1 mil calculation is correct, as is your 22X/32X = 0.6875 mil. To obtain an interdot arc estimate of 0.5 mil, you'd need to dial up to 44X (i.e. object doubles in size with respect to the reticle interdot arc (distance)>>>means double the mag from 22X). Halving the mag to 11X would increase the interdot arc (distance) to 2.0 mil.

It might be easier to think about this as though you were looking through the scope at a ruler edge. What will happen to the apparent reticle interdot arc (distance) as the ruler image changes in size? The reticle remains a constant size to your eye...the object (the ruler) does not. As the ruler image increases in size (increasing mag), the reticle interdot arc (distance) decreases proportionally. As the ruler decreases in size (decreasing mag) the reticle interdot arc (distance) increases proportionally.

3.44 MOA/mil is a correction factor that is only applied after you first determine the mag-dependent change in reticle subtension in units of mils, which you did correctly. So for the 1 mil (22X/22X) calculation, the corresponding value in MOA would be 3.44 MOA (1.0 mil x 3.44 MOA/mil = 3.44 MOA); for the 0.6875 mil (22X/32X) calculation, the corresponding value would be 2.37 MOA (0.6875 mil x 3.44 MOA/mil).

This illustrates a problem with scopes that have different units for the reticle subtensions and turret values: if your turrets are in MOA and the reticle in mils, you have to make these kinds of calculations in order estimate a required turret adjustment based on the reticle subtensions. The units aren't the same, which means a PITA trying to do the math in your head every time...it's too easy to make mistakes that way. Regardless of whether the reticle subtensions are in MOA or mil, if the reticle units match the turret units, no conversion is necessary...the adjustment you see in the reticle is what you dial on the turret. If you are in this position, at a minimum I would suggest very generous rounding, for example, use 3.5 instead of 3.44. It's rough, but if you have 0.25 MOA turrets, it's easier to convert the turret adjustment estimate using 3.5 (i.e. 14 clicks on a 0.25 MOA turret). For many applications, that estimate will be close enough to hit the target, but not nearly as challenging to figure out in your head on the fly.

Very thorough -- but it makes my head hurt!!

 

[mikecr]

There is another issue I've noticed with NXS scopes; when you adjust the eyepiece to get reticle focus -you change the scope's magnification.
For me, an 8x32 at '32x' and reticle focused is only 25x. This, matching my focused 8.5x25 Mk4s at 25x.
So I gain nothing in an NXS 32x scope except extra weight, and MOA dials that do not actually adjust in accurate MOA increments (like a Mk4 does).

 

[fm1947]

I got the formula from night force and put it in a spreadsheet works great