Rifle Scopes First/Second focal plain... educate me:)

[srzbiznz]

Can you guys please clear this up for me.... I have a Millett LRS-1 which has a Second focal plane reticle and I have been reading up on it, but I am sorta confused. My understanding is that with a second focal plane reticle, if I sight in at 100 yards on 10x, then move to 200 yards and make the adjustments but change the power to 12.5x, it will be hitting higher/lower than if it was left at 10x? Where as a first focal plane reticle will hit the same regardless of magnification? Sorry if this is confusing, but I'm a noob:/

If this is correct, would I just have to shoot it at a single power at all distances or is there a formula to calculate the deviations? If not, I may be sending this scope back and paying a little more:O Thanks.

 

[Lindy]

Re: First/Second focal plain... educate me

On a properly made SFP scope, if you zero at 100 yards, and then move to a greater distance, dialing the appropriate elevation for that distance, the center crosshairs will be on target no matter what magnification you choose.

An example: My 500 yard dope is 3.4 mils. If I am using a SFP scope with 3.4 mils dialed, I will hit the target point-of-aim/point of impact at any magnification.

With an SFP reticle, the value of the marks on the reticle change with magnification. Say, for example, that my 600 yard dope is 4.4 mils, and I want to dial 3.4, then hold over 1 mil to hit a 600 yard target. There is only one magnification for an SFP scope where that will work. With most such scopes, it is the highest power the scope will dial to.

On a SFP scope set with the reticle correct at 20 power, dialing down to 10 power, each 1 mil mark in the reticle will now cover 2 mils.

That may not matter for the kind of shooting that you're doing.

You may find this link useful: Sniper's Hide Optics FAQs

 

[XTR]

Re: First/Second focal plain... educate me

A SFP reticle stays the same size as you magnify the sight picture. Think about it, the think about that, the picture gets bigger, the lines stay the same distance apart so the location on the target changes.

In a FFP reticle is magnified with the sight picture so the lines stay in the same location in relation to the size of the picture.

Ranging reticles in SFP scopes are designed to work at one power setting (not always but usually the highest). If you are looking at something far away you're probably on high power. If you know that and know how it works you are fine. All the Nightforce NXS scopes are SFP, and people love them. Unless you are doing a lot if reticle holdoffs you're fine.

 

[srzbiznz]

Re: First/Second focal plain... educate me

Thanks for the quick replies! I know that my scope can be used at 25x and also 12.5x to calculate ranges, I believe at 25x it's 1 mil and at 12.5x it's .5 mil.

The range I go to only goes out to 200 yards but I will also be shooting out to/past 1000 yards every once in a while, so I just wanted to be sure that my scope can manage those ranges.

So I think you're saying that the bullet will hit the same spot no matter what magnification it is on, but it has to be on a certain magnification for ranging distances?

 

[CMonty256]

Re: First/Second focal plain... educate me

A FFP reticle will remain constant in size relative to the target at all magnifications (Turn up the zoom and the reticle gets bigger)

A SFP reticle will only remain constant in size relative to the shooter at all magnifications (Turn up the zoom and the reticle remains the same)

 

[CMonty256]

Re: First/Second focal plain... educate me

Heres what that does for us:

On a FFP scope, when a target is measured on the reticle as being 1.0 mils wide at 3x mag, it will remain 1.0 mils wide at all magnification levels. Which is why its so useful for tactical purposes, a target can be ranged through measuring at any magnification.

Whereas on a SFP scope the reticle is generally calibrated to be accurate at maximum magnification. Therefore that previously 1.0 mil wide target would only appear to be 1 mil wide at the top, say on a 1-10x, it would be 10x. When you dialed down the soon to 5x your target would be twice as large on the reticle as before, indicating 2 mils wide. Dial down to 2.5x and your target would appear to be 4 mils wide.

It is possible to do anything a FFP reticle can do with a SFP, You just need to calculate a correction factor based off of magnification level versus calibrated magnification

"My target is shows as 4 mils wide, I'm at 75% of my max magnification, therefore the target is 3 mils"

But it gets tricky in real life when the target is 2.8 mils and you know your magnification is somewhere between the 5 and the 8 dot. Where in a FFP scope 2.8mils is always 2.8 mils.

Hope this helps

Monty