Second Focal Plane Question

[xswanted]

Odd question and I believe I know the answer but......

If you take a 5-20 SFP scope that has a reticle that subtends to 1 moa at full power (20x)

Would the reticle subtend to 2 moa at 10x

OR

Would the reticle subtend to 2 moa half between the top and bottom power at 12.5x?

 

[Kracken McGraw]

Yes, 2 moa at 10x and 4moa at 5x.

 

[xswanted]

Yes, 2 moa at 10x and 4moa at 5x.

Yep, exactly what I’ve found.

For the sake of argument though why wouldn’t it be the other way?

 

[std7mag]

Because SFP scopes usually have the reticle designed for full power. In this case 20.
Doesn't matter the magnification range, or lowest power.

 

[Ned Ludd]

Yep, exactly what I’ve found.

For the sake of argument though why wouldn’t it be the other way?

It's all about how much the reticle occludes or covers up (subtends) at a given magnification. In a 2nd focal plane scope, the reticle doesn't change size as the magnification changes. In other words, the reticle itself always stays the same size to your eye, while the image behind it increases or decreases as you change the magnification. If the reticle subtends 1 MOA with the magnification set on 20X, the image in the scope will decrease in size (to your eye) by half at 10X, and by a factor of four a 5X, all while the reticle stays the same size. That means the reticle will subtend 2 MOA at 10X (i.e. double) and 4 MOA at 5X (i.e. quadruple) because the size of the image behind it decreased as the magnification decreased.

All you really need to know with a 2nd focal plane scope is the magnification at which it is calibrated. Sometimes it may be at full magnification, but often it is not. Regardless, as long as you know the mag at which the scope reticle is calibrated, you simply divide the calibration magnification by the actual magnification setting to determine the reticle subtension. For example, scope is calibrated at 20X>>> reticle hashmark width = 1 MOA. For 10X: calibration mag/actual mag x calibration subtension>>> (20X/10X) x 1 MOA = 2 MOA.

 

[std7mag]

Also be aware that changing magnification on a SFP scope can sometimes throw point of impact off some.

And that if it's calibrated for say 20X, that your magnification ring may not be exactly on 10X to get your 2 MOA subtension.
Much like most paralax adjustments may be off some at a given range.

 

[Tempest]

My youngest son looks like Ted Nugent. Yes, needs one really bad!

 

[rardoin]

My youngest son looks like Ted Nugent. Yes, needs one really bad!

A SFP scope or a hair cut? Wrong thread perhaps.

 

[Tempest]

A SFP scope or a hair cut? Wrong thread perhaps.

You are correct!

 

[rardoin]

You are correct!

Well....you could buy HIM a scope....and just permanently borrow it! Haircuts rate below a nice scope, don't they???

 

[K22]

Also be aware that changing magnification on a SFP scope can sometimes throw point of impact off some.

And that if it's calibrated for say 20X, that your magnification ring may not be exactly on 10X to get your 2 MOA subtension.
Much like most paralax adjustments may be off some at a given range.

Just curious, I sometime dial down in high mirage situations when hunting ground hogs, i.e 16x to 8 or 10X. I've check POI at these lower magnifications at the range and noticed no changes in POI. But I'm not dialing up or down on the scope turrents or using the any reticle subtens to adjust for elevation / windage compensation. Reticles are standard duplex and all scopes are second focal plane.

PS: Most of the time I shoot only at the highest magnification of the scope it's only in rare high mirage conditions that I dial the power down to shoot.

 

[std7mag]

I didn't say always.
In some cases, with some scopes, it has been known to happen.

 

[SSL]

Slightly off the original topic; all my scopes are SFP and nearly all have either mildots or hash marks. I never considered the difference in dot size relative to power setting, but I struggled with the value of each dot for distance at various powers. My solution was to use the Strelok Pro app to give me a visual of each value. It is a great app that is pretty accurate as long as you enter the correct info for each load/velocity/BC and etc. I then made scope setting stickers to paste inside my rear scope cap for easy viewing. I mark each dot/hash mark with true zero (which is a constant) and the value of each subtend at both maximum magnification and a good mid-range magnification. EXAMPLE: On a 6-18X, I usually mark the yardage values at both 6 and 18 power or 10 and 18 depending on the rifle in question. Even scopes designed for specific cartridges (Like the Bushnell Drop Zone offerings) can be used with other calibers by running them through Strelok.

 

[Rob01]

One thing to remember is that 10x on the power ring might not be exactly 10x. If you do plan to use the reticle for holds then you would be wise to go and actually check it and you might find it might be off so mark the power ring where it would be right. Same with 5x.

 

[SSL]

One thing to remember is that 10x on the power ring might not be exactly 10x. If you do plan to use the reticle for holds then you would be wise to go and actually check it and you might find it might be off so mark the power ring where it would be right. Same with 5x.

Quite true. This is where actually shooting comes into play. Regardless of how accurate any ballistic chart or app is, the shooter has to verify with their own rigs. Far too many variables to rely on just plain numbers.

 

[Lefty Trigger]

Because SFP scopes usually have the reticle designed for full power. In this case 20.
Doesn't matter the magnification range, or lowest power.

Not all are this way, the magnification (power ring) normally has one power setting marked and that is the power setting the reticle is ranged for. Look for a dot or a hash mark next to a power setting number. I have one SFP scope that goes to 25 power but the reticle is ranged accurately at 18 power. However the reticle does not change in size like a FFP scope.

 

[Tempest]

You are correct!

The strange part when I went back and looked at this... I never clicked on this topic. I rarely if ever visit the scope topics.

I replied to the other topic???

 

[Old Navy]

Also be aware that changing magnification on a SFP scope can sometimes throw point of impact off some.

Interesting. Never gave that any thought. I think I will check that next time out. Hopefully, tomorrow.

 

[284winner]

Quite true. This is where actually shooting comes into play. Regardless of how accurate any ballistic chart or app is, the shooter has to verify with their own rigs. Far too many variables to rely on just plain numbers.

In a way, I actually miss the days before ballistic calculators showed up. I'd spend all day at the range and enjoy learning the final load I developed that shot the best. Sure, today it shortens the process and needs less range time so maybe good in some ways but before ballistic calculators was still...The good ole days.

 

[SSL]

In a way, I actually miss the days before ballistic calculators showed up. I'd spend all day at the range and enjoy learning the final load I developed that shot the best. Sure, today it shortens the process and needs less range time so maybe good in some ways but before ballistic calculators was still...The good ole days.

I would agree, but no matter how sophisticated the app is, it can't be trusted unless it is verified with actual extensive shooting. Just running the numbers and trusting the accuracy of your settings is going to be frustrating. I'm afraid too many will go that route rather than spending the time and effort to test those settings before trusting them on game. Those programs or apps are tools, but they can't do the whole job for you.