FFP Scope Education

[MADCleveland]

I am trying to educate myself of the mechanics of FFP scope operation. I understand the reticle magnifies with the target. What I don't understand is the difference in shooting from a SFP scope when the magnification is adjusted. On a SFP, once I zero the gun at a specific magnification, I must shoot at that magnification to maintain that zero. My question: Since the FFP scope magnifies the reticle and the target simultaneously, is the zero valid for all magnification settings?

I do realize that I have to adjust for bullet drop at longer distances and other variables, but I'm unclear on the operational differences between FFP and SFP at the several magnification settings.

Thanks.

 

[rijndael]

On a SFP, once I zero the gun at a specific magnification, I must shoot at that magnification to maintain that zero.

If the scope's zero changes with a magnification change then it's broken.

In SFP, the reticle size is fixed and zooming just changes the target's size. With FFP, they both change together.

 

[SSL]

In answer to your question, your reticle and any other aiming points (hash marks, mildots and etc.) are valid at all magnification settings with FFP scopes, assuming proper configuration and quality. For example, if your scope is properly zeroed at 100 yards and the next hash mark down was designed and verified for 200 yards, they will both be accurate regardless of the power setting.

 

[Turbulent Turtle]

I am trying to educate myself of the mechanics of FFP scope operation. I understand the reticle magnifies with the target. What I don't understand is the difference in shooting from a SFP scope when the magnification is adjusted. On a SFP, once I zero the gun at a specific magnification, I must shoot at that magnification to maintain that zero. My question: Since the FFP scope magnifies the reticle and the target simultaneously, is the zero valid for all magnification settings?

I do realize that I have to adjust for bullet drop at longer distances and other variables, but I'm unclear on the operational differences between FFP and SFP at the several magnification settings.

Thanks.

I do not understand what you mean by the bolded text. In my SFP scopes, when I zero at a specific magnification, the zero remains throughout the zoom range. In fact, that is the principle of operation for a variable scope.

 

[SSL]

I do not understand what you mean by the bolded text. In my SFP scopes, when I zero at a specific magnification, the zero remains throughout the zoom range. In fact, that is the principle of operation for a variable scope.

That is a good point. With either SFP or FFP scopes the primary aiming point (we'll assume a crosshair) will be the same regardless of magnification. The ancillary markings will change values with magnification changes with SFP but remain the same with FFP.

 

[MADCleveland]

I do not understand what you mean by the bolded text. In my SFP scopes, when I zero at a specific magnification, the zero remains throughout the zoom range. In fact, that is the principle of operation for a variable scope.

At long distance, 1000 yards, it is my understanding that SFP requires a correction due to the difference btw the target size and reticle.

 

[rijndael]

At long distance, 1000 yards, it is my understanding that SFP requires a correction due to the difference btw the target size and reticle.

If you use the SFP scope at the power on which it was calibrated, often max power (or marked some way), then the reticle subtension is correct at any distance - whether that's 100 or 1000 yards.

As a side item, you should always double check that the marked/calibrated magnification is properly marked. Sometimes it's off by a hair.

 

[MADCleveland]

That is a good point. With either SFP or FFP scopes the primary aiming point (we'll assume a crosshair) will be the same regardless of magnification. The ancillary markings will change values with magnification changes with SFP but remain the same with FFP.

Thanks. That is the correct statement I intended. Btw, you know Dr. Bob?

 

[SSL]

Thanks. That is the correct statement I intended. Btw, you know Dr. Bob?

Sorry, I'm not familiar with him.

 

[MADCleveland]

Thanks. That is the correct statement I intended. Btw, you know Dr. Bob?

So, with SFP, how does one determine the value of the other subtension values at different magnification settings?

 

[rijndael]

It's math.
1/2 Calibrated Power = 2X the reticle value
1/4 Calibrated Power = 4X the reticle value

For me, I do almost everything at full or half power.

 

[Delfuego]

At long distance, 1000 yards, it is my understanding that SFP requires a correction due to the difference btw the target size and reticle.

This is incorrect.

You might be mingling information and confusing yourself. Either scope will maintain your zero (point of aim) at any distance (crosshairs or dot). SFP scope reticles, like mildot, are generally only accurate at max power, similar to a fixed power scope. A FFP reticle's subtensions are accurate at "any power" because the reticle scales in size with magnification. This applies to reticle markings (subtensions) above/below & left/right of the crosshairs/dot. The crosshairs/dot should not change.

A good question is what are you using your scope for? There is a right and wrong tool for every job. What is the job of this scope?

 

[MADCleveland]

This is incorrect.

You might be mingling information and confusing yourself. Either scope will maintain your zero (point of aim) at any distance (crosshairs or dot). SFP scope reticles, like mildot, are generally only accurate at max power, similar to a fixed power scope. A FFP reticle's subtensions are accurate at "any power" because the reticle scales in size with magnification. This applies to reticle markings (subtensions) above/below & left/right of the crosshairs/dot. The crosshairs/dot should not change.

A good question is what are you using your scope for? There is a right and wrong tool for every job. What is the job of this scope?

Well, I have a FFP scope that I'd like to mount on a .308 Axis to shoot target long distance. But I want to thoroughly understand the difference in operation between SFP and FFP beyond the FFP reticle scaling in size with magnification. Why are SFP reticles only accurate at max power? What happens when scaling down from max power to make a SFP inaccurate?

 

[MADCleveland]

Many shooters do appreciate second focal plane designs because the sight picture is always consistent. However the most significant disadvantage of SFP scopes when compared to FFP is that any advanced features like bullet drop compensation and ranging can only be calibrated to be accurate at one magnification setting. With very few exceptions, BDC and ranging in SFP scopes are calibrated to be accurate at the highest magnification setting. It makes sense; the highest power magnification is likely where you’ll be taking a long-range shot anyway, so that is where those features are most useful. However, if you have ever lost the opportunity to take a good shot because that trophy 10-point ducked under some brush while you were fiddling with your optic, you can understand the potential upsides of being able to range estimate and account for bullet drop at any magnification.

 

[Delfuego]

Because the subtensions are static in a SFP scope. The image changes size, the reticle remains the same. They are not "inaccurate" they they just change their relative value or measurement.

1mil = 1mil at 24x power (max )
1mil = 2mil at 12x power (1/2)
1 mil @ 14.5 power? Who the f*** knows?
I don't have time to do that kinda math

From the post above, it sounds like you already have it all figured out. I though you came here to ask a question. You are either over thinking, or already had your mind made up and are looking for validation. The best way to figure this out is with more time behind a scope and less time on the internet. I am sorry for trying to help. Delfuego out...

 

[MADCleveland]

Many shooters do appreciate second focal plane designs because the sight picture is always consistent. However the most significant disadvantage of SFP scopes when compared to FFP is that any advanced features like bullet drop compensation and ranging can only be calibrated to be accurate at one magnification setting. With very few exceptions, BDC and ranging in SFP scopes are calibrated to be accurate at the highest magnification setting. It makes sense; the highest power magnification is likely where you’ll be taking a long-range shot anyway, so that is where those features are most useful. However, if you have ever lost the opportunity to take a good shot because that trophy 10-point ducked under some brush while you were fiddling with your optic, you can understand the potential upsides of being able to range estimate and account for bullet drop at any magnification.

I guess I'm asking why BDC and ranging on a SFP aren't accurate at less than full magnification.

 

[MADCleveland]

Because the subtensions are static in a SFP scope. The image changes size, the reticle remains the same. They are not "inaccurate" they they just change their relative value or measurement.

1mil = 1mil at 24x power (max )
1mil = 2mil at 12x power (1/2)
1 mil @ 14.5 power? Who the f*** knows?
I don't have time to do that kinda math

From the post above, it sounds like you already have it all figured out. I though you came here to ask a question. You are either over thinking, or already had your mind made up and are looking for validation. The best way to figure this out is with more time behind a scope and less time on the internet. I am sorry for trying to help. Delfuego out...

Thanks. That makes sense.

 

[SSL]

I guess I'm asking why BDC and ranging on a SFP aren't accurate at less than full magnification.

Because that's how the manufacturer made and calibrated them.

 

[KY_Windage]

That is a good point. With either SFP or FFP scopes the primary aiming point (we'll assume a crosshair) will be the same regardless of magnification.

The crosshairs, like the parts of the reticle, get fatter as you turn the power up. I don't consider that to be staying the same, and it is one of my principle objections to FFP for target shooting.

 

[KY_Windage]

At long distance, 1000 yards, it is my understanding that SFP requires a correction due to the difference btw the target size and reticle.

No such correction is necessary.