Educate me on SFP POI and magnifaction changes

[Jmccracken1214]

Im looking to get a 3-15 pst viper HD SFP and never had a SFP.

If I zero this optic, with changing zoom mag, since it has dialing turrets, my center crosshair wont be affected, correct...? Its only the holdover hash marks that will be changed with different levels of zoom?

If I zero at 100, on 15x... and then need to take a shot at 300 yards at 10x and I dial the correct adjustment, I should be able to just hold dead on and shoot, right?

 

[db2000]

This is my basic understanding.

Because the reticle size through the lens changes with magnification, it does affect ranging with hash marks. The scope manufacturers designate which magnification the hash marks will have a set value.
You can zero at any magnification.
It does not change how much you dial. You’re correct, just dial appropriately and aim center of crosshairs.

 

[_Windrider_]

What you said is basically correct.

however, one of the big limitations of second focal plane is that you generally have to zero at max magnification or minimum magnification because those are the only true magnifications that you know are accurate on the markings on the scope you could dial 10 on the scope, but you will never truly know if you’re at 10 because it may not be 100% perfect and if it’s not 100% perfect it’s gonna throw off your holdovers.

 

[mjh30]

Zero at any magnification, the difference should be miniscule with the vortex, not enuff to worry about. Hold overs and ranging should be on max power with this scope. Zero at 10x, dial for 300 at 15x you should be spot on.

 

[Aftermath]

Zero is still zero at every magnification. I usually zero at max magnification so I can really see where I am aiming and use a good, solid rest to zero with. That zero is still a good zero when I zoom to minimum. Or any zoom in between.

The zoom level changes the holdover yardages tho.

For example, I have a Swarovski Z5i 3.5-18x44P with the BRX reticle. I have it on a 280 Ackley that is pushing 140 gr Berger Hunting VLD at 3250 fps. I have it zeroed at 275 yards. I used the factory app to start with but then went and verified with actual shots on target. The app was real real close. I have attached screenshots from that app for you showing max, mid and min zooms. I have a piece of cardstock taped to my stock with the verified yardages written using these 3 zoom levels on a mockup reticle. Most of the time, deer are well within my point blank zero and I just shoot them. Once in a while, they are far enough away that I want to verify with the LRF. Then I look for that number on my chart, maybe change the zoom accordingly.

 

[TheOE800]

Zero is zero. POI shift from one magnification to another is merely an inaccurate/gross zero.

 

[Scott_at_Vortex]

Im looking to get a 3-15 pst viper HD SFP and never had a SFP.

If I zero this optic, with changing zoom mag, since it has dialing turrets, my center crosshair wont be affected, correct...? Its only the holdover hash marks that will be changed with different levels of zoom?

If I zero at 100, on 15x... and then need to take a shot at 300 yards at 10x and I dial the correct adjustment, I should be able to just hold dead on and shoot, right?

Yes to all all of your questions.
Just out of curiosity did you get a 3-15 PST or a Viper HD 3-15?


v/r
Scott

 

[Jmccracken1214]

Yes to all all of your questions.
Just out of curiosity did you get a 3-15 PST or a Viper HD 3-15?


v/r
Scott

The viper hd

 

[koshkin]

Zero is zero. POI shift from one magnification to another is merely an inaccurate/gross zero.

That's not quite accurate. All variable power SFP scopes, every single one in existence, exhibit zero shift to some degree as you change magnification. High quality scopes have so little shift that you can't easily see it. I have seen some lower end ones with very significant shift. It is also harder to keep this zero shift contained with high erector ratio scopes.

It is a different phenomenon than the change in the subtensions of the reticle with magnification.

ILya

 

[TheOE800]

Must be my misunderstanding then. IIRC, zero at max power should translate to zero at minimum power or that’s how I was taught anyway. Zero the 3-9x deer scope at 9x.

 

[koshkin]

Must be my misunderstanding then. IIRC, zero at max power should translate to zero at minimum power or that’s how I was taught anyway. Zero the 3-9x deer scope at 9x.

That is the desired goal of a well built SFP scope. For a minute of deer, that is generally true for nearly any good quality SFP scope.

When you adjust the magnification, there is always some slight lateral shift of the lenses, so the image hops around slightly. The reticle is superimposed onto the image after all that happens, so depending on where you are in the magnification range, it will be superimposed on a slightly different part of the image.

With most well built scopes it is well within the rifle disspersion. In the past, I would occasionally see something that would shift nearly a mrad between high mag and low mag.

ILya