"Circular" polarizer that is only "circular"?

[DanK]

This happens to involve CPLs for drones, but the question is a general one.

Recently, I began experimenting a bit with drone photography. Unfortunately, the filter manufacturers with which I am very familiar and that I trust don't make many drone filters, so I had to pick blindly from the available ones.

I wanted a CPL for obvious reasons. Frewell is one of the major suppliers of drone filters, so I bought theirs. It's a rectangluar metal plate (to fit the front of the camera) with a circular glass filter that rotates. (Not clear how useful rotation is if the drone is 100m up in the air and moving, but that's another issue.)

When I looked closely, it seemed not to be a CPL at all.

First, my understanding is that CPLs have two glass plates, a linear polarizer on the front and a quarter-wave panel on the back. The Freewell filter seems to have only a single piece of glass. Second, unlike CPLs, when rotated in front of a polarized light source, the Freewell has the same darkening regardless of whether viewed from front or back. CPLs show the darkening primarily when viewed through the back (that is toward the lens).

I wrote the retailer, who replied only "Freewell Circular Polarizer CPL Filter for DJI Air 2S is intended to be a single lens with the ability to rotate via the red threading."

Am I wrong that this can't be a CPL? I suspect it doesn't matter; I assume the camera uses contrast-detection AF and that a linear polarizer (which I think this is) should be fine. Still, I'd like to know. If it's a linear polarizer that happens to be circular in shape (as were my linear polarizers back in the days before AF), they shouldn't be marketed as CPLs.

NiSi also produces DJI filters, and while I know nothing about their filters, I know that some of their other gear is considered reasonable quality. I was finally able to get a NiSi polarizer for the drone, and sure enough, it doesn't rotate and is labeled PL, not CPL.

[rpcrowe]

I have a drone but have never flown it. However, I would suspect that using a polarizing filter of any type could be counter productive when flying a drone. Obviously when using any polarizing filter the angle of that filter to the sun is of optimum importance. However, the drone will fly in any direction and therefore the polarization obtained by the filter will change as the drone's flight path changes.
Wouldn't this cause changes in color and density of your drone shots - often while you are in the midst of a shot?
I may be wrong and I suspect the answer may be contained in one of these YouTube videos...
https://www.youtube.com/results?sear...larizer+filter

[DanK]

I have a drone but have never flown it. However, I would suspect that using a polarizing filter of any type could be counter productive when flying a drone. Obviously when using any polarizing filter the angle of that filter to the sun is of optimum importance. However, the drone will fly in any direction and therefore the polarization obtained by the filter will change as the drone's flight path changes.
Wouldn't this cause changes in color and density of your drone shots - often while you are in the midst of a shot?
I may be wrong and I suspect the answer may be contained in one of these YouTube videos...
https://www.youtube.com/results?sear...larizer+filter

A different question, but I partly agree. The polarization would definitely change as orientation changes. I do only single shots, not videos, so I'm not worried that much about changes of that sort. More important is that drones have wide-angle lenses, so any shot of a blue sky is likely to be inconsistent if a polarizing filter is on. However, the way I use my drone, the sky is rarely the issue. The issue is unwanted reflections, primarily from water, and a polarizing filter is the only way to reduce that problem.

The first of the videos you linked supports my hunch. The polarizer he shows is two sheets of glass, and the darkening varies as you rotate one of them. That isn't the case with the Freewell.

[lunaticitizen]

Hmm... smells like false representation to me.

[Manfred M]

Dan - my understanding is that the difference between a linear polarizer and a circular polar is that a circular polarizer combines a linear polarizer as a front element and has a quarter wave plate filter bonded to the back of it. You have a single unit rotating together as one of the axes of both the linear polarizer and quarter plate have to line up exactly. The rotation of either a liner or cpol is there so that the photographer can dial in the effect that they are looking for.

I have never heard of Freewell, but NiSi is a highly regarded filter producer that produces first rate products. I look at NiSi in the being in the same class as B+W, Heliopan, Formatt-Hitech, etc.

[DanK]

Dan - my understanding is that the difference between a linear polarizer and a circular polar is that a circular polarizer combines a linear polarizer as a front element and has a quarter wave plate filter bonded to the back of it. You have a single unit rotating together as one of the axes of both the linear polarizer and quarter plate have to line up exactly. The rotation of either a liner or cpol is there so that the photographer can dial in the effect that they are looking for.

I have never heard of Freewell, but NiSi is a highly regarded filter producer that produces first rate products. I look at NiSi in the being in the same class as B+W, Heliopan, Formatt-Hitech, etc.

Manfred,

Ahh, that may be the answer. I'd forgotten that the two plates in a circular polarizer rotate together. so they could appear as one piece of glass (particularly given how thin the glass in drone filters is). However, my understanding is that CPLs aren't reversable. That is, you get the full darkening with rotation only when the light passes first through the linear polarizer. I just checked this with two CPLs from reputable brands, a Hoya and a Marumi. I used a pair of polarized sunglasses to obtain a polarized light source. In both cases, rotating them in the proper orientation yielded one position that was extremely dark, as it should. Rotating them after reversing them didn't; there was no point in the rotation at which the image was greatly darkened. The Freewell acts like a linear polarizer in this test: it seems to make no difference whether I reverse the filter.

Dan

[Manfred M]

Manfred,

Ahh, that may be the answer. I'd forgotten that the two plates in a circular polarizer rotate together. so they could appear as one piece of glass (particularly given how thin the glass in drone filters is). However, my understanding is that CPLs aren't reversable. That is, you get the full darkening with rotation only when the light passes first through the linear polarizer. I just checked this with two CPLs from reputable brands, a Hoya and a Marumi. I used a pair of polarized sunglasses to obtain a polarized light source. In both cases, rotating them in the proper orientation yielded one position that was extremely dark, as it should. Rotating them after reversing them didn't; there was no point in the rotation at which the image was greatly darkened. The Freewell acts like a linear polarizer in this test: it seems to make no difference whether I reverse the filter.

Dan

You are correct on the Cpol; it is needs to be oriented so that the light passes through the linear polarizer before it passes through the quarter wave plate, so your test confirms that the Freewell is not a cPol.