Where is the mark on the polarizer supposed to be?

Started 2 months ago | Questions thread
Gerry Winterbourne Forum Pro • Posts: 17,518
Re: Where is the mark on the polarizer supposed to be?

Dem Bell wrote:

Most polarizers, either linear or circular, have a mark on their rim that is supposed to tell which orientation produces maximum effect.

Correct. Natural light vibrates at all angles; polarising tries to remove all light but a particular angle. (I say "tries" because if it really cut out all light except one specific angle it would darken things by 359/360 - almost pitch black.) The mark on the filter shows how that angle is aligned.

The sun is in the plane of the filter, the mark on the rim points towards the sun - bang, maximum effect. That's what I thought.

It's not just the light from the sun that is polarised - so is light from all over the sky, reflections from the scenery (and remember that unless you put the light source in the frame, all the light reaching the camera is reflected).

In practice, therefore, there's unlikely to be a single angle of maximum effect. The index mark is just a guide to your starting point.

I understand that you don't always want the maximum effect, that not all polarizers have a mark, that you can see the effect of the polarizer in the EVF, etc.. etc... My question is not about how to use a polarizer but about the meaning of the mark on its rotatable part. Surely it has something to do with the way the tiny parallel wire grid is oriented in the filter, right?

Well, there's no wire grid so in that sense you're wrong. But substitute "angle of polarisation" for wire grid and you're right.

There are some nice on-line tutorials like this:

https://learn.zoner.com/learn-to-use-a-polarizing-filter/

that confirm that and show the following example:

Three polarizing filters. All the filters have a mark on their rotatable part that marks the polarization plane. (Two have a line; the one on the right has a small triangle.) [from https://learn.zoner.com/learn-to-use-a-polarizing-filter/ ]

What they say makes perfect sense, except this does not seem to be the case in practice. It is easy enough to determine which orientation of the polarizer completely removes reflections from a window or blocks all light coming from an LCD screen. You don't even need to attach the filter to a camera.

As it happens, I had three random old Hoya polarizers in the draw, and I checked if the mark on the rim of these filters corresponds to the orientation in which the polarized light is blocked. No, it does not! It is all over the place! For two filters it was off by about 15-20 degrees, the third one was off by about 60 degrees. The actual position of the mark has nothing to do with the orientation of the polarizer grid and its maximum effect.

This begs the question: why bother putting a mark on the rim if it does not mean anything? Is this mark just a visual cue to tell you by how much the polarizer is being rotated? I am confused now. Should the tutorials be rewritten? Can my three filters be duds?

Three polarizing filters. The orientation that removes all polarized light (this is the orientation perpendicular to the direction of the polarizer grid) is marked by white putty (UHU White Tack). For all three filters, this is not the orientation marked on the filter.

No filter removes all polarised light. First, if it did the image would be black; second, polarisation works at more than one angle. The effect you see depends on many things; unless the scene is identical each time - which means that clouds don't move, no time passes, foliage is perfectly stationary etc - you wouldn't expect to see the same effect every time.

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Gerry
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I'm happy for anyone to edit any of my photos and display the results
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First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne
gerry.winterbourne@ntlworld.com

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