FNG - Second Focal Plane question

[Slateman]

Hello everyone. Long time lurker, first time poster

Question in regards to using a SFP scope. I understand that when you zero the rifle, you zero it at that magnification only. My question is, how do you compensate for this? Is there a mathematical formula?

So, for example, if I have a 4-14x SFP scope and I zero it at 14x, how do I adjust if the scope is at 4x/8x/Any other magnification?

 

[dddoo7]

zero is zero regardless of the magnification. The only thing that changes is the spacing of the reticle. If you have a mil-dot reticle, the spacing will only be accurate at one magnification (usually max). The zero does not change with magnification

 

[Graham]

The zero should not change with magnification.

 

[Greg Langelius *]

I understand that when you zero the rifle, you zero it at that magnification only.

Not so. Zeros should be right on at the given distance, regardless of magnification.

Magnification affects MilDot ranging with SFP scopes. Ranging should be done at a given magnification only, usually specified in the scope instruction pamphlet.

Read the scope instruction manual until you are sure you understand what its trying to convey.

Greg

 

[Graham]

Magnification can affect the zero of a SFP scope, but it shouldn't.

 

[Slateman]

zero is zero regardless of the magnification. The only thing that changes is the spacing of the reticle. If you have a mil-dot reticle, the spacing will only be accurate at one magnification (usually max). The zero does not change with magnification

Okay, so then help me understand

If I'm zeroed at 14x magnification at 300 yards, how do I adjust at 8x magnification at 500 yards? I know that I must allow for "x" number of inches and that that drop equates to "x" number of mils at 14x magnification. How would I convert that at another magnification?

 

[dddoo7]

There is no good way to use your reticle for hold overs except at max (or whatever it is) magnification. I have all second focal plane scopes, but I either leave them at max magnification or I dial the mil's on the turrets. If you dial the drop on the turrets then you can be at any magnification and be correct.


--Daniel

 

[Graham]

FNG - Second Focal Plane question

There is no good way to use your reticle for hold overs except at max (or whatever it is) magnification.

Yes there is: You can calibrate the half-power point. That is, provided the reticle subtends correctly against the hard stop. Otherwise you will have to derive a unique multiplier for the scope.

 

[dddoo7]

Yes there is: You can calibrate the half-power point. That is, provided the reticle subtends correctly against the hard stop. Otherwise you will have to derive a unique multiplier for the scope.

Yes... But that also assumes you get the power ring precisely adjusted each time.


--Daniel

 

[Graham]

Yes... But that also assumes you get the power ring precisely adjusted each time.

Thats why, when you calibrate the half-power point, you mark it externally on the scope tube.

 

[JBNj]

Okay, so then help me understand

If I'm zeroed at 14x magnification at 300 yards, how do I adjust at 8x magnification at 500 yards? I know that I must allow for "x" number of inches and that that drop equates to "x" number of mils at 14x magnification. How would I convert that at another magnification?

Take 14 divide by 8= 1.75 now multiply that with 3.6"=6.3" per mil dot on your reticle at 8x magnification at 100 yards.
Now say we are shooting a 178 amax at 2650. Zeroed at 300. To get to 500 will require 35.5" of elevation(variables aside) Now take your 6.3 per mil, multiply by 5(first number of yards ex. 5=500, 1=100, 10=1000,etc)= 31.5. Now take your drop 35.5 divide by 31.5= 1.126mils. For our sake round it 1.1 mils for your correction. And that is why ffp is better no fumbling with math needed.

If I'm wrong point it out i could use a good lesson.

 

[1TallTx'n]

I have a Vortex Viper PST 6-24 x 50 EBR1 MOA/MOA that I'll soon be putting to use and the paper work that came with it says - with actual subtension of 2 MOA reticle space

24x = 2 MOA
12x = 4 MOA
8x = 6 MOA
6x = 8 MOA

I hope this helps, if not, well, at least I added to my post count

 

[djskit]

Slateman - All the points above are valid and true. But let me address your question from a different angle.

Zero your rifle at 100 yards: That means, you've adjusted the turrets such that the bullet impact is in the same spot as the center of the cross hairs. Then you will loosen your turrets, and adjust them to "0" and tighten.

No matter what power you are on, that center is unchanged (unless you have a POS scope, which is another story). So, at 100 yards, you bullet should go right at the center of the cross hairs, no matter what power you are on.

Now we go back to 300 yard. It seems you are conflating 2 distinct elements of operating the scope. Dialing the turrets and hold-overs using the reticle. Let's take them one at a time:

Turrets: At 300 yards, you can dial the turret to your 300 yard dope (for me it's 1.1 mil). Then you should be able to hold center and the bullets will hit point of aim - regardless of the power setting.

Hold over: You can also leave your turret at "0" and hold 1.1 mils using the reticle. However, this will only work at the designated power setting for the SFP scope - for yours, that should be 14x. If you put it on a different power setting, a 1.1 mil hold will not be 1.1 mils. If you feel ambitious, you can calculate the values of different power setting and confirm via calibration etc.

This is why FFP scopes are so popular (but more expensive) the reticle subtends the same distance at any power setting. 1.1 mils will work at 300 yards at any power setting.

 

[Slateman]

Hold over: You can also leave your turret at "0" and hold 1.1 mils using the reticle. However, this will only work at the designated power setting for the SFP scope - for yours, that should be 14x. If you put it on a different power setting, a 1.1 mil hold will not be 1.1 mils. If you feel ambitious, you can calculate the values of different power setting and confirm via calibration etc.

This is why FFP scopes are so popular (but more expensive) the reticle subtends the same distance at any power setting. 1.1 mils will work at 300 yards at any power setting.

This is exactly what I'm trying to figure out. How do you determine what the hold is at a different magnification? If your hold is 2.5 mils at 14x, what would it be at 8x?

 

[PlinkIt]

if you are going to take the time to do that much math why not just make the necessary adjustments to the turrets?...

the other simple solution is first focal plane.. no math required..

Use a scope that reflects the methods you wish to use it.. use turrets on second focal or get a first focal for multiple magnification holdovers..

Sometimes its just better to keep it simple

 

[JBNj]

This is exactly what I'm trying to figure out. How do you determine what the hold is at a different magnification? If your hold is 2.5 mils at 14x, what would it be at 8x?

Edit divided wrong. Answer is 1.4 mils at 8x

2.5x3.6=9"@100
14/8=1.75
1.75*3.6= 6.3" per mil at 8x magnification
Take the drop of 9"/6.3= 1.428 mils at 8x magnification
1.4 (rounded to nearest 10th) mils at 8x is the same as 2.5 mils at 14x

 

[Graham]

FNG - Second Focal Plane question

Just use 7x. But first find out where it really is on the magnification ring.

 

[JBNj]

I would do as graham says and index your scope at 7x instead of trying to fumble with the math. This way whatever your at 14x it would be divided in half for mils at 7x. 2.5 mil at 14x. Equals 1.25 mil at 7x

Take a 36" target and place it at 100yards. It should appear at 10 mils on your reticle. Now adjust your magnification till it shows 5mils. Mark your scope as this will be 7x mag.
Work smarter,not harder.

 

[Slateman]

if you are going to take the time to do that much math why not just make the necessary adjustments to the turrets?...

the other simple solution is first focal plane.. no math required..

Use a scope that reflects the methods you wish to use it.. use turrets on second focal or get a first focal for multiple magnification holdovers..

Sometimes its just better to keep it simple

I don't disagree. But I'd like to know how to use one should the need arise. And SFP scopes are usually cheaper will still providing

Edit divided wrong. Answer is 1.4 mils at 8x

2.5x3.6=9"@100
14/8=1.75
1.75*3.6= 6.3" per mil at 8x magnification
Take the drop of 9"/6.3= 1.428 mils at 8x magnification
1.4 (rounded to nearest 10th) mils at 8x is the same as 2.5 mils at 14x

Let's back up a second.


(Zero magnification/ current magnification) * (14x adjustment * 3.6) = per mil at current magnification
Drop/wind in inches / per mil at current magnification = mil hold at current magnification

And if I zeroed at 300 yards, I would simply substitute 10.8 for the 3.6?

So, if I hold at 2.5 mils at 300 yards it would be 2.5 * 10.8 = 27in
14/8 = 1.75
1.75*10.8 = 18.9 inches per mil at 8x
27/18.9 = 1.428
1.4 mils at 8x is the same as 2.5 mils at 14x? Will the mils stay the same regardless of range?

And how do I convert all of this to MOA?

 

[JBNj]

"1.75*10.8 = 18.9 inches per mil at 8x"

The above Will only apply at 300 yards. Mil value will change based on yardage @ 8x.
Example.
@8x magnification; 6.3" per mil @100, 12.6" per mil @200, 18.9" per mil [MENTION=89035]300[/MENTION], and so on. Otherwise you seem to get it.

I take it you have a mildot reticle,moa turrets?

 

[Graham]

Stop converting to inches.

 

[wigwamitus]

I have a LP 3.5-10x TMR ... I use it only on 3.5x or on 10x ... usually only on 3.5x to maximize FOV.

After much shooting (er calibration) it seems dividing by 3 works, out to 325yds. I think the "real" magnifications on this scope are 3.3 up to 9.9 ... maybe the marketing folks rounded those off to 3.5 to 10.

Where I have found this useful is shooting .22LR or 7.62 subs (similar drop profile). I can crank up to 200yds and then walk to 325yds. My AB calculator will tell me something between 7.2 and 7.4 mils of adjustment are required, I won't have enough clicks left to handle this. At 10x I would have to hold 7.2 to 7.4 ... but at 3.5 I divide by 3 and need to hold 2.4 to 2.466 which seems easier for me on TMR reticle.

The only time I use 10x is 600yds, otherwise I can see the targets ok at 3.5x.

I don't have to struggle to find where 10x or 3.5x are because they are at the opposite extremes of the magnification dial.

 

[Slateman]

"1.75*10.8 = 18.9 inches per mil at 8x"

The above Will only apply at 300 yards. Mil value will change based on yardage @ 8x.
Example.
@8x magnification; 6.3" per mil @100, 12.6" per mil @200, 18.9" per mil [MENTION=89035]300[/MENTION], and so on. Otherwise you seem to get it.

I take it you have a mildot reticle,moa turrets?

Right, but I will know the hold at max magnification based on my data card.

I did. I got rid of it. Never again. It will be MOA/MOA or mil/mil. I'm leaning more towards MOA/MOA

 

[JBNj]

Stop converting to inches.

At first i was like whhhaaat? But i see your point now after looking at it.

 

[Graham]

FNG - Second Focal Plane question

LOL! I have that effect on people.