Calibrating a CPL Filter Min/Max Positions

[Waddizzle]

Okay, so you rotate a CPL filter to vary the amount of filtering. I've never seen one that's marked with designations showing where the rotation is for maximum and minimum filtering. I have found a crude, yet seemingly highly effective, way of positioning the filter very near the min/max filtering positions. Attach one to your lens, and then look through your camera at a flat screen LCD TV, which is turned on, of course, while you rotate the filter. My question is simple. What is the best practice, if there is one, for marking your filter's min/max positions? I've been tempted to use nail polish.

[RobertTheFat · Accepted solution]

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@Waddizzle wrote:
Okay, so you rotate a CPL filter to vary the amount of filtering. I've never seen one that's marked with designations showing where the rotation is for maximum and minimum filtering. I have found a crude, yet seemingly highly effective, way of positioning the filter very near the min/max filtering positions. Attach one to your lens, and then look through your camera at a flat screen LCD TV, which is turned on, of course, while you rotate the filter. My question is simple. What is the best practice, if there is one, for marking your filter's min/max positions? I've been tempted to use nail polish.

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There are no min/max positions. The degree of polarization of the incident light depends on the angle whereat the sun shines upon the subject. The best practice is to rotate the filter until the sky is at its darkest and/or it appears suitably dark for your purposes. If instead you're photographing a pool of water, the ideal position is when you see the correct amount of reflection (possibly none) therein. It really is that absurdly simple: what you see is what you get; and if you don't like it, change it.

But the fact that the angle makes so much difference means that polarizers don't work all that well with wide-angle lenses. From one side of the picture to the other, the angle can be quite different. So the sky can appear half blue and half white.

Yes, some of the laws of physics should be changed or repealed. But that outcome is unlikely in our lifetimes, given the partisan political bickering wherewithal we are cursed. So we have to learn to live with things as they are, alas, rather than as we might wish them to be.

[Waddizzle]

Hmm, I had considered the possibility that the direction of the light could be a factor, but sunlight is randomly polarized, so I thougth that the direction the light comes in shouldn't make a diffference. Hmm, thanks. BTW, turning my CPL filter 90 degrees can completely blank out an LCD TV, which leads me to believe that my circular polarizer is actually a linear polarizer, because the the light coming out an LCD TV is vertically polarized. Maybe, I'm thinking on it too much, thanks again, though.

[RobertTheFat]

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@Waddizzle wrote:
Hmm, I had considered the possibility that the direction of the light could be a factor, but sunlight is randomly polarized, so I thougth that the direction the light comes in shouldn't make a diffference. Hmm, thanks. BTW, turning my CPL filter 90 degrees can completely blank out an LCD TV, which leads me to believe that my circular polarizer is actually a linear polarizer, because the the light coming out an LCD TV is vertically polarized. Maybe, I'm thinking on it too much, thanks again, though.

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Wellll... The word "circular" has its meaning stretched a bit in this context. Optics isn't my field; but as I understand it, a circular polarizer is actually two linear polarizers in series, the object being to avoid confusing the camera's autofocus mechanism, which evidently relies on polarized light. So I guess you could read "circular" as "all the way around to where you started".

[Waddizzle]

Thanks, Bob from Boston. I've done some research into the physics of sunlight. As i recalled from my college physics classes, sunlight is randomly polarized.

However, sunlight can become less randomly polarized when it is reflected off of an object[s], such as back scatter from the atmosphere [the bright blue sky].

[TCampbell]

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@RobertTheFat wrote:

Wellll... The word "circular" has its meaning stretched a bit in this context. Optics isn't my field; but as I understand it, a circular polarizer is actually two linear polarizers in series, the object being to avoid confusing the camera's autofocus mechanism, which evidently relies on polarized light. So I guess you could read "circular" as "all the way around to where you started".

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Two linear polarizers can go together and rotate to become a "variable neutral desnity" (not recommended on very wide angle lenses).

A single "linear" polarizer works well for manual focus cameras when the photographer has a seperate light meter.  But it can create problems with accuracy for cameras that have built-in metering (where it can alter the way the meter perceives the light and trick it into metering incorrectly) and also make it difficult for phase-detect auto-focus systems (the beam splitter in the phase detect AF system has problems with polarized light).  BTW ... it's not much of a problem for cameras using constrast-detect AF (cameras which use "live view" to focus - because there is no beam-splitter involved.)

A "Circular" polarizer solves these problems.  It's actually just an ordinary "linear" polarizer... but adds an extra layer called a "quarter-wave plate"   This allows the primary "linear polarizer" to cut the unwanted reflections... but what comes out the other side is polarized light.  But the polarized light then hits the quarter-wave plate which somewhat alters the polarization effect so that it not longer creates as much trouble for auto-focus and metering systems.

Polarizing, in general, works because of the difference between "scattering" surfaces vs "reflecting" surfaces.

If you shine light onto a wall painted with white (flat / non-glossy) paint, then that surface is a "scattering" surface.  Meaning that when the light hits the surface it bounces off in such a way that it scatters randomly in all directions.  But if that were a glossy surface (say a glass-coated or mirrored surface) then you'd get a "reflection" in which the light reflects off the surface on the same angle in which it hit the surface.  So if I point a flashlight at a mirror on a 30º angle, then the light bounces off in a 30º angle (but going the opposite direction and the difference bewteen the "angle of incidence" and the "angle of reflection" will have a 60º difference (30º + 30º = 60º).

But something else happens... because the light hit a reflective (non-scattering) surface, the light was also polarized by that mirror-like surface.  

A polarizer has an orientation.  Light in the correct orientation can pass through uneffected.  But light polarized in a direction which is orthogonal to the polarizer will be blocked.  Since the reflections on your subject have a mostly common polarity, it's possible to tune the filter to the orthogonal direction to block that light... but light polarized in any other direction (typically caused by "scattering" surfaces) will pass through with little effect.  

If I'm taking a photo of a leaf that has a waxy surface, the leaf itself is "green" but the waxy surface creates a reflection.  If you could strip off the waxy coating, what you'd have is a green "scattering" surface.  So the polarizer allows you to eliminate the reflections (caused by the waxy surface) and see the true "green" of the leaf (created by the "scattering" surface.)  This causes the polarizer to appear to "saturate" colors (the colors are actually true -- but now you're seeing true color and reflections and glare (which wash out the true color) are eliminated.

There is an exception where polarizers do not work well... if the light appears to originate from either directly ahead of you OR if the light is originating from near directly BEHIND you, then the reflection will not take on any specific polarity (you need the light to reflect at an angle to get it to take on a direction of polarity).  In these situations you'll find that the polarizers don't work.  They work best when the source of the reflection is coming from an stronger angle.

Here's a link to an article which describes how a "circular" polarizer works... but you could certain do a web search and find many more articles:  http://www.apioptics.com/circular-polarizers.html

[Waddizzle]

Thanks for the info.  That's pretty much what I had surmised about the behavior of CPL filters and sunlight.  Loved the explanation of difference between scattering and reflective surfaces.