Rifle Scopes "Tunneling"

[brand692]

Can someone explain to me why this "phenomenon" is more prevalent in FFP scopes? I'm not entirely void of knowledge when it comes to the working of scopes, but after going through my different scopes, I realized this only plagued my FFP scopes.

I never remember seeing it on any of my NF SFP scopes, any Leupold, and I have several SFP scopes here that don't "tunnel."

Is there one brand that is better than others? I don't have a S&B or Hendsoldt, and only recently acquired a PH (its not even here yet). Or is it something that is just part of the game with FFP scopes?

EDIT:
Lindy, I'm sure you have a link for me, but it is rather late, and I'm sure you have retired for the evening.

Also for those of you wondering what I'm talking about, I may have mislabeled what I'm referring to. At a point during magnification adjustment from high to low, at least on all my USOs, as you come further and further down, there is a point where the FOV no longer gets larger, but the magnification power continues to become smaller. I've heard it referred to before as "tunneling" but don't know if that is the official name of whats going on.

 

[Jon A]

Re: "Tunneling"

The correlation you have noticed between "tunneling" and FFP scopes is coincidental. One thing has nothing to do with the other. Aspects of the optical system design other than where the reticle is placed are responsible.

 

[brand692]

Re: "Tunneling"

Thanks Jon. Is there a FFP that doesn't tunnel?

 

[dcjs]

Re: "Tunneling"

What is happening when you see "tunneling" is that the opening in the optical system where the reticle sits in a FFP scope is physically too small to allow full field of view. In a FFP scope, this is simply the edge of the reticle, in a SFP scope it is the opening of the front of the erector tube.
Since a reticle has to be installed at the front of the erector in a FFP scope, there is less space there to provide a large opening. The reticle has to be put in place using lockrings, and there has to be additional space for centering the reticle on the erector. In simple terms, the crucial area in a SFP scope is just a hole, while in a FFP scope you have to install something in that hole, adjust it and lock it in place to be shock proof. This requires more space and thus will provide a smaller free diameter to look through, all else being equal. So it is indeed harder to design a FFP scope without the tunneling effect, and things get more complicated the higher the zoom ratio is.

So why not just make that diameter bigger? The erector basically is a tube moving inside a tube, and with a certain main tube diameter, the erector tube diameter cannot be increased if one wants to allow a certain amount of movement (equaling reticle adjustment range). So FOV on low power, adjustemnt range and main tube diameter are correlated, and it is an engineering challenge to maximise bot FOV on low power and adjustment range in a given main tube diameter.

Good news is, your soon-to arrive Heritage is free from this tunneling effect because the internals are generously sized despite the huge adjustment range. FOV will grow all the way down to low power despite the 5x zoom ratio.

 

[Jon A]

Re: "Tunneling"

Thanks for the correction, David. I didn't realize the reticles would be installed in a manor that limited the size of the opening. I guess I made one too many assumptions. It was based upon scopes that are available in both FFP and SFP (IOR 2.5-10, 3-18, etc) in which both models show similar tunneling. Now I'm confused as to why their FFP's don't tunnel a bunch more than their SFP's.

 

[brand692]

Re: "Tunneling"

Thanks for the explanation. It makes sense now why I'm only seeing it in my FFP scopes. I've never had a IOR so I can't attest to how they perform.

Do the Hensoldts and S&Bs tunnel as well?

How about the Vortex Razor HD or 35mm tubed USO? Do the 35mm USOs use a larger erector tube therefore reducing the "tunneling" effect?

 

[dcjs]

Re: "Tunneling"

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Jon A</div><div class="ubbcode-body">Thanks for the correction, David. I didn't realize the reticles would be installed in a manor that limited the size of the opening. I guess I made one too many assumptions. It was based upon scopes that are available in both FFP and SFP (IOR 2.5-10, 3-18, etc) in which both models show similar tunneling. Now I'm confused as to why their FFP's don't tunnel a bunch more than their SFP's. </div></div>
My explanation was of course a generalization and I can't speak for the particular designs other manufacturers use. Of course it's no problem to make a SFP scope tunnel, much less than making a FFP scope <span style="font-style: italic">not</span> tunnel.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: brand692</div><div class="ubbcode-body">Thanks for the explanation. It makes sense now why I'm only seeing it in my FFP scopes. I've never had a IOR so I can't attest to how they perform.

Do the Hensoldts and S&Bs tunnel as well?

How about the Vortex Razor HD or 35mm tubed USO? Do the 35mm USOs use a larger erector tube therefore reducing the "tunneling" effect? </div></div>
Look at the FOV figures to get an idea wether a particular scope will tunnel and how much. If the magnification factor is 5x as in a 5-25x scope, the FOV on low power should be 5x as big as on high power. Just divide the FOV number on low power by the number on high power and look at the number you get. If it's smaller than the zoom factor (high magnification number divided by low magnification number), you will see tunneling, because that is an indicator of the FOV not growing all the way to lowest magnification as you decrease power.

 

[brand692]

Re: "Tunneling"

I received the PH today, and as you stated David, it doesn't have any tunneling.

I should have known to simply use the math to verify the amount of tunneling. Didn't even cross my mind. Thanks for the help.

 

[koshkin]

Re: "Tunneling"

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: David S.</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Jon A</div><div class="ubbcode-body">Thanks for the correction, David. I didn't realize the reticles would be installed in a manor that limited the size of the opening. I guess I made one too many assumptions. It was based upon scopes that are available in both FFP and SFP (IOR 2.5-10, 3-18, etc) in which both models show similar tunneling. Now I'm confused as to why their FFP's don't tunnel a bunch more than their SFP's. </div></div>
My explanation was of course a generalization and I can't speak for the particular designs other manufacturers use. Of course it's no problem to make a SFP scope tunnel, much less than making a FFP scope <span style="font-style: italic">not</span> tunnel.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: brand692</div><div class="ubbcode-body">Thanks for the explanation. It makes sense now why I'm only seeing it in my FFP scopes. I've never had a IOR so I can't attest to how they perform.

Do the Hensoldts and S&Bs tunnel as well?

How about the Vortex Razor HD or 35mm tubed USO? Do the 35mm USOs use a larger erector tube therefore reducing the "tunneling" effect? </div></div>
Look at the FOV figures to get an idea wether a particular scope will tunnel and how much. If the magnification factor is 5x as in a 5-25x scope, the FOV on low power should be 5x as big as on high power. Just divide the FOV number on low power by the number on high power and look at the number you get. If it's smaller than the zoom factor (high magnification number divided by low magnification number), you will see tunneling, because that is an indicator of the FOV not growing all the way to lowest magnification as you decrease power. </div></div>

That is a very lucid explanation, David, and easily the best on I have seen on the subject!!!

However, if you do not mind, I will add a comment.

Your explanation fits higher end scopes very nicely. With some less expensive designs I have seen interesting tricks done.

For example, in order to avoid the increase in tunneling effect at low magnification, I have seen the field of view limited elsewhere in the scope which throttles the achievable field of view at high magnification. That is kinda the worst of both worlds: you add something that decreases overall performance of the scope in order to avoid fixing the original problem.

ILya