Tunneling is a phenomenon that some variable power scopes exhibit when the magnification is turned down below a certain level. As the magnification is reduced, and before tunneling occurs, the field of view through the scope increases—and that’s what probably most users expect. If a scope is subject to tunneling, though, at some point when the magnification is reduced, the field of view no longer increases. The magnification continues to decrease, but the outer limits of the field of view close in.

When tunneling occurs what that means is we don’t get the benefits of an entire scope magnification range. For example let’s say our 5-25× scope starts tunneling when the magnification is reduced to 6 power and continuing beyond that point the image becomes smaller, the first focal plane reticle becomes thinner and harder to see, but we don’t get any increase in field of view which would normally be the advantage that offsets the first two characteristics. In that case rather than having a scope with a one to five magnification range that we no doubt paid for, we have a scope with what is essentially a one to four magnification range.

The fact that (some?) Nightforce scopes exhibit tunneling is something I learned only after seeing it mentioned on another Internet site. Having a 7-35× ATACR model it was easy enough to check, and sure enough: The field of view stopped increasing when the power was dialed down to about 8.5 to 9 or so. That means that in terms of field of view I have a scope whose magnification range is about one to four, not one to five as 35 divided by 7 would indicate.

As I say, it was only a very brief Internet post that prompted me to check my scope, and considering how popular and as highly-regarded as Nightforce scopes are, that is a puzzle: Why isn’t that characteristic more widely mentioned? Does it apply to only one or a few models?

At this point someone will probably be asking, “So what? Whoever needs the maximum field of view of a variable power scope anyway? Does anyone ever shoot that way?”

Well, the answer of course is that it depends on how the scope is being used. If a sniper or hunter has the magnification turned down as far as possible to permit scanning a larger overwatch area, then yes, it matters. She may increase the power if it’s time to shoot, but if we pay for a one to five magnification range, that’s what we should get.

Anyway as I say, it’s a strange semi-secret that my particular Nightforce scope exhibits tunneling, and strange because we’re not referring to a $79.95 Amazon special. My question therefore is what other scopes exhibit tunneling when the power is turned down to a certain level? Have you ever checked yours? Would it matter if it did occur?
(And for whatever it’s worth my Leupold scopes I checked and my March scope don’t “tunnel.”)

Thanks for all replies.

I haden't heard of this so I had to google it. It appears to be related to the size of the objective lens, and some scopes do it while others don't.

quote:

Originally posted by SgtGold:
It appears to be related to the size of the objective lens ....

Where did you find that?
At first blush it would seem to make sense, but the below list contains scopes with 56mm objectives (like the Nightforce 7-35×56mm ATACR I have) on both sides of the question.

TUNNELING PRESENT

Hensoldt ZF 3-12x56mm SSG-P: tunneling between 3-4x, absent beyond that.
IOR 2.5-10x42mm: 2.5-4x
IOR 3-18x42: 3-4x
IOR 3.5-18x50mm: 3.5-4x
IOR 4.5-14x50mm SFP: 4.5-7x
Schmidt & Bender 5-25x56mm PM2: 5-7.25x
U.S. Optics SN-3 1.8-10x37mm: 1.8-2.25x
U.S. Optics SN-3 3.2-17x44mm: 3.2-5x

TUNNELING ABSENT

IOR 6-24x56mm (2012 model)
Kahles K624i Gen1 & 2 6-24x56mm
Leupold Mark4 3.5-10x40mm M3
Nightforce ATACR 5-25x56mm*
Steiner 5-25x56mm (all Steiners?)
SWFA SS HD 5-20x50mm
Vortex Razor HD 5-20x50mm
Zeiss Victory Diavari 6-24x56mm T*

https://www.snipershide.com/sh...ation-ranges.203938/

It’s a question that has piqued my curiosity now and I would be interested in any explanation for the phenomenon.

* Added: The claim in the one Internet post I found that the Nightforce 5-25 ATACR does not exhibit tunneling is disputed in another statement I found.
If anything, the different claims demonstrate that anyone who is interested in the question should confirm it for himself, and not rely on Internet claims.

This message has been edited. Last edited by: sigfreund,

This is the last post in the linked thread:

"Tunneling is usually caused by manufacturers trying to get more zoom range than the erector can support. A bunch of Schmidts do this as well. Essentially towards the lower end of the mag range you stop getting a change in FOV and while the image gets smaller (appears to be zooming out). Its likely a design issue rather than a manufacturing issue."

My initial comment about objective size came from a video on the subject, but that looks like it might have been incorrect.

Thanks for that link.

The more I think about it, the more I wonder why it doesn’t get more discussion.

I suppose I shouldn’t be surprised, but there are countless Internet reviews of various scope sights and virtually none of them mention tunneling. And the same is true of the size of the “eye box” or vision cone. Yeah, if you’re 25 years old and do all of your shooting from the well-settled prone or a bench, then it’s not an issue, but 50+ years later and you’re still trying to shoot from awkward positions it makes a difference.

Those things are far more important to me than whether there’s a bit of color fringing at the corner of a white wall or even if the image gets a little soft at the edge of the visual image. I guess people who decide to get into making YouTube reviews probably feel that they have to follow the same formulas as everyone else.

Thanks sigfreund I never knew such a thing existed, so out of curiosity I checked a few I could easily get to.

March 10-60 no tunneling

SWFA HD 5-20 no tunneling

SWFA HD 1-6 no tunneling

Sightron SIII 6-24 no tunneling

Sightron SII Big Sky adjustable objective no tunneling (No longer made)

Kahles 2-7 no tunneling

Kahles 3-9 no tunneling

Thanks for checking that list.

My US Optics FDN 1.8-10x shows it just barely when you slide back into 1.8x.