The question is at the end if someone doesn’t want to read all the preliminary discussion.

Some time ago I posted questions about using a polarizing filter with a scopesight to eliminate reflections from glass. Although that’s not something a hunter or competitor would ever need, it could be valuable for the professional, especially a law enforcement sniper. As much to examine the concept as anything, I decided to look into options. There is a commercial offering but in addition to being fairly expensive (~$100), I’m not too enamored of the concept because of features that I believe would be unhandy unless it were necessary to use it all the time. I decided, therefore, to explore making ordinary photographic polarizing filters work.

The advertising for the above unit cautioned against putting a filter over the objective (front) lens of the sight due to the possibility of causing point of impact shifts. Based on my own experiments, I doubt that that’s something to actually be concerned about, but it seemed there were other disadvantages to objective lens filters. The rifle I wanted to conduct the experiments with is equipped with a Leupold Mark 6 3-18×44mm scope, and I use Vortex lens covers on both ends. (They are excellent products, BTW, and far superior to others I’ve used.) I measured the eyepiece cover and concluded that a 46mm filter would be a close fit. There was no way to screw the filter onto the eyepiece cover, but I assumed I could figure out some way to attach for the times it would be useful.

As it turned out, the female side of the filter fits snugly over the eyepiece cover extension and stays in place by friction:



The cover can’t be closed with the filter in place, so if I were moving around with it on the rifle, I’d secure it with a bit of electrical tape.

Using the filter does make a difference for viewing some targets.

Without the filter in place, the reflection on the windshield makes it impossible to see in. (Yes, I realize that if the driver was the target, he’d be back in the seat and visible through the side window, but this is just a demonstration so please bear with me.)



With the filter in place.



So, my experiment appears to be a success, and at $22 for the filter, much less expensive—and more convenient—than using the commercial device.

The Question.
When a polarizing filter is usually used either on the front of a camera lens or just holding and looking through it, it’s necessary to rotate it to find the proper angle to eliminate something like the above windshield reflection.

When I put the filter on the eyepiece lens of the scopesight, though, the windshield reflection is eliminated regardless of its orientation. Rotating it makes no difference in the image, and my question is Why? I can speculate about how the light is affected by passing through the sight, but nothing seems to make sense to me. Do any of our optics authorities have an explanation?

Is it one of the polarizer filters which has two separate pieces that rotate against each other? If so, if you have what would normally be the objective side of the filter stuck on the scope and are rotating the threaded part that would normally attach to the camera lens, then you're just rotating a metal collar, not the polarizer itself.

If you are certain you are rotating the filter then either you bought a circular polarizer, which is not orientation sensitive, or it is changing and you just didn't notice. (Or possibly there is something odd going on inside the scope, but based on my understanding of optics, I don't think so.)

quote:

Originally posted by maladat:
[T]hen you're just rotating a metal collar, not the polarizer itself.

Ah, ha. I didn’t think about that, but you’re right, I was just turning the collar. I pulled the filter off a few times and thought I’d positioned it differently, but evidently not. When I checked again and made sure the polarizer was rotated, then I could see the reflection change through the scope.

Although circular polarizers seem to be much more common these days, I made sure to order a linear type because I wasn’t sure if a circular would have the same effects.

Thanks for your insight in clearing up the mystery.

Glad I could help.

I can't imagine ever personally needing one, but I can see how it would be useful for a police sniper.

What a remarkably interesting project!

After years of never having used them, I eventually started fly fishing with polarized sunglasses and was shocked at the difference they made in cutting down on water surface glare and successfully locating fish... but I somehow never thought about how effective a polarized lens could be on a rifle scope.

Thanks for posting Sigfreund!

While you are experimenting try looking at some distance targets and see if the lens has any affect on seeing through the mirage. Everything I've seen says no but I have seen no actual experiments on this. Plus you can save me some money conducting my own experiments unless you have luck with yours.

quote:

Originally posted by low8option:
While you are experimenting try looking at some distance targets and see if the lens has any affect on seeing through the mirage.

That’s another interesting question. I’ll definitely try to remember to check the next time I have an opportunity.

I like to pass these things on even though I realize most shooters would never have any practical use for the information. I enjoy learning the unusual and assume there are at least a few others like me.

My guess is that you will actually see more mirage because of the additional f-stop brought on by the polarizer increasing the depth of field. On the other hand, it may take the edge off some of the scintillating seen in cases of heavy mirage.

quote:

Originally posted by sigfreund:
I like to pass these things on even though I realize most shooters would never have any practical use for the information. I enjoy learning the unusual and assume there are at least a few others like me.

This is applicable to anyone shooting an optic in competition. Since CMP started allowing optics I use my spotting scope a lot less. Anything I can do to make my rifle optic more useful is fine by me.

When I transitioned from Service Rifle & Match Rifle to F-class, I put aside my spotting scope because I could clearly see the targets in my riflescope to see the score and read mirage.

At first my riflescope was a 10X, then 20X and then 36X. When I switched from .223 to .308 and built a dedicated match rifle, the new scope went to 42X. At that point, I pulled out my old spotting scope and then upgraded it to a Kowa a year later.

I have my Kowa right beside me on the firing line where I can transition from it to my riflescope in a second or two. I now spend much more time on the Kowa and looking at flags, then looking through my 50X riflescope. The mirage through my riflescope can be quite different from what I see in my Kowa.

Now that CMP allows optics on the rifles, I think lots of folks will eschew the use of their spotting scopes. Then again, the maximum magnification allowed is 4.5X, at least for SR, so that should still work. But get much higher magnification, especially at LR and you will see people bringing back their spotting scopes to the line.

This thread tickled my memory so I went to check on my optics equipment and as I riffled through my set of filters, I spotted a larger than usual case. It was an 86mm CPL, perfect for my Kowa spotting scope. A quick check in my records showed that I bought it 2 years ago and totally forgot about it. Age.

Anywho, I have a three day match coming up this weekend and I'm going to play with the CPL on the spotting scope. I also have a camera adapter for the Kowa so with a little luck I should be able to shoot some HD video through the Kowa with and without the CPL. That should show us something about what it does with mirage.

quote:

Originally posted by NikonUser:
That should show us something.

Looking forward to it.

It's definitely dark. I'm guestimating at least one f-stop, maybe two. I'll try to meter through the camera and confirm those values. That will be important data.

Interesting post, thanks for sharing.

I learned a lot about polarizing filters during various geology courses (notably optical mineralogy) and used the hell out of them in one of my other passions (fly fishing), but have never given much thought to their use in long range shooting or in another life-threatening scenario.

Will be waiting for results. Even if no pictures just your opinion about results will be very beneficial.

I pulled out a light meter this morning and measured the value of the CPL. It's a full stop, meaning it cuts the incoming light by half.

And looking back at the pictures in the OP, you can see the second one is darker.

What's even funnier is that I finally remembered that I had bought that CPL specifically for testing the Kowa in mirage conditions. This was at the time I was playing with yellow filters on my riflescope. When I finally bought the CPL, I was engrossed in some other testing issue and getting ready for the Nationals with the team, and totally forgot about it. Every time I thought about continuing my testing with filters, I kept thinking about buying a CPL for the Kowa but never did and then forgot about it. Good thing that, else I would have several of them now.

This growing old shtick is really getting old. Let's see if I remember between now and Sunday evening.

quote:

Originally posted by NikonUser:
My guess is that you will actually see more mirage because of the additional f-stop brought on by the polarizer increasing the depth of field. On the other hand, it may take the edge off some of the scintillating seen in cases of heavy mirage.

Correct me if I'm wrong, but the only way you'd see an increased depth of field is if the decreased light through the scope resulted in increased pupil dilation in your eye.

Depth of field in camera lenses is really a direct result of aperture size rather than actual light transmission, right? (E.g., a camera lens at F/2.8 and the same lens at F/2.8 with a 4-stop ND filter will have the same depth of field, but 16 times the exposure length.)

An excellent point, and to be honest I'm not sure anymore. The problem is that I'm transferring photographic context to riflescope applications. But in a camera, I have control over the aperture, something I do not have in a riflescope.

Using ND filters allows me to use a wider aperture in the camera on a bright day and that helps with controlling depth of field. But I can't do that in the riflescope.

So you may be right in that it doesn't change the DOF, but just makes the image darker. Which is what you see in the OP images.

Thanks for pointing that out. I need to reflect on this (pun intended) some more.

On further thought, I think I got misled by the jump in f-stop since we use ND filters to be able to increase the aperture size in bright sunlight in order to shrink the DOF and I extended that in my mind by increasing DOF since we can't control aperture on the riflescope.

maladat, curse you for making me thing further on this.

So, if the DOF does not change with the CPL, then if it does anything at all to the mirage, it will certainly not add to it. Now, I'm really looking forward (another intended pun) to seeing if it does anything this weekend, and here I am wishing for heavy mirage during a competition. How sad is that?

The weather forecast for this weekend is not good, but that supposed to hit Saturday, so I can still try it on Friday.