How to set camera angle and rotating ring without seeing what the camera sees
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Last updated July 27, 2004
Polarizing filters (AKA polarizers) are far and away the most important and frequently used filters in the outdoor photographer's arsenal and can be indispensable indoors as well. Polarizers add richness and clarity to photographs by suppressing two all too common forms of unwanted lightatmospheric scatter and specular reflections, including glare off water, glass and even foliage. The samples below illustrate the polarizer's effect.
Table Notes: Camera angles chosen for maximum polarization of unwanted light in all samples; automatic exposures; no post-processing other than resizing.
To my mind, there are no polarizer advantages unique to digital cameras, but digital cameras with limited dynamic range benefit greatly from the selective suppression of excess contrast. Due to the limited UV sensitivity found in most digital cameras, polarizers also provide a welcome and effective alternative for haze control at favorable camera-sun angles.
What, No TTL Control?
The direct through-the-lens (TTL) view of a single lens reflex (SLR) camera simplifies polarizer use considerably: Adjust your aim and the rotating ring of your polarizer for the desired effect under direct visual control and shoot. The same goes for cameras with well-shaded electronic viewfinders (EVFs). But these cameras are still vastly outnumbered by the digital rangefinders, which provide real-time TTL control only via their main rear LCDs.
Here's the rub: In practice, you can't always see these rear LCDs well enough to fine-tune your polarizer technique. That's all too often the case with my Oly C-2020Z in bright sunlight—just when I might need a polarizer the most. To guarantee workable TTL control with a rangefinder in all conditions, you'd have to carry an LCD shade. I have one, but frankly, I hate to futz with it.
So how do you use a polarizer on a rangefinder camera without TTL control? Well, there are several possibilities:
Or your could keep things quick and simple with the "blind" polarizer technique described next.
Well, You Needn't
Fortunately, there's a much faster and easier way to get the most from a polarizer without relying on anything other than your own eyeballs and an understanding of how ambient light gets polarized. You'll have to learn to think like the enemy (unwanted scatter and reflections, or more specifically, the polarized components of these phenomena), but once armed with a certain practical understanding of polarization, you'll be able to determine accurately and reliably the only 2 things you need to know to use your polarizer to maximum effect:
With that information in hand, the strategy for maximum scatter or reflection control becomes straightforward:
There's nothing wrong with a direct look through the LCD or dismounted polarizer to
You can also confirm polarizer effect with your camera's TTL metering system. But now that I've become one with polarization, I seldom feel a need to peek. The blind method works very well in nearly all cases.
It's Just A Tool
Before going any further, let me emphasize that the polarizer, like any other accessory, is simply a means to an end, with benefits and costs that must be balanced against many others in the shot at hand.
In low-light situations, the exposure hit (at least 2 stops with most polarizers) may well be untenable. Even in decent light, the 2+ stops of potential shutter speed lost to the polarizer will make camera shake that much more likely in handheld shots, especially telephotos.
You'll often find it inconvenient, impossible or undesirable to shoot in or even near a target direction. And there will be many times when the polarizer will predictably have no beneficial effect whatsoever. In such circumstances, removing the polarizer will reclaim at least 2 stops of light, clean up your optics and reduce the chance of flare.
Bottom line: Whatever the constraints, the ability to predict target direction and polarization and likely polarizer effect will save you time and help you make the most of the photographic choices at hand—whether or not you have TTL control, and whether or not you end up actually using a polarizer.
To use a polarizer effectively without TTL control, you must first find and if necessary mark your polarizer's blocking axis, which may or may not align with a white dot or other symbol on the polarizer's outer rotating ring. Figuring out where the most strongly polarized light's coming from and the direction of its polarization is the next step. For the filter to have maximum effect, the blocking axis of the polarizer must align closely with the polarization of the unwanted light.
To block atmospheric scatter,
So, if the sun is 30° above the horizon in the west, you should be aiming north or south with the top of the blocking axis 30° east of vertical.
To block reflections off water or glass,
At 30° and wider angles (i.e., at 35 mm equivalent focal lengths under 85-100 mm), polarizer coverage becomes limited by deviations from the target direction across the camera's field of view. This predictable fall-off in polarizer efficacy most strongly affects peripheral scene elements more than 15° off the target direction—e.g., sky well off the arc of maximally polarized skylight or specular reflections well off Brewster's angle. A prime example of the former is the sky banding seen in wide-angle big-sky shots early and late in the day.Uneven suppression of reflections can be seen to some extent in the glare examples shown above.
Besides minimizing scatter and glare, you can use your polarizer:
The neutral density application requires the polarizer to be set for minimum effect, which is also easily be done without TTL control. The remaining techniques typically require full TTL control for best results, but they are beyond the scope of this article.
In the absence of polarized light, a polarizer is nothing more than a 1- to 2-stop neutral density (ND) filter. If an ND effect isn't what you're after, a polarizer won't do you any good in unpolarized light.
Poorly Polarized Scatter
If you find yourself shooting within ~30° of the line between you and the sun, either toward or away, a polarizer won't help you much with atmospheric scatter. Skylight coming from those directions acquires very little polarization in the scattering process.
But Glare off Foliage May Still Benefit
That said, note that many leaves and grasses turn out to be surprisingly good specular reflectors. Even if the sky is hopeless, a polarizer can often cut a rewarding amount of color-sapping white glare off foliage under these circumstances, especially when shooting close to the sun.
Poorly Polarized Glare
Reflections off very dull or metallic surfaces are almost always lost causes—there just won't be enough polarization to work with.
When controlling glare off a non-metallic specular reflector like water or glass, the farther you are off Brewster's angle (generally ~55°) from the normal to the offending surface, the less effective your polarizer will be. At 90° to Brewster's angle, reflections will be completely unpolarized—and thus immune to your polarizer. This is often the case when shooting opposite the sun, with either water or foliage glare, as illustrated in the scatter samples below.
But Foliage and Water May Still Benefit
When shooting near the sun, however, glare off foliage or water with waves may well be coming at you from a wide range of angles, some of which may still be profitably blocked.
When in doubt, try it out, particularly if you're shooting toward the sun. Even if you can't see the effect in your LCD or electronic TTL viewfinder, if there's a profit to be made, your meter will register a light loss as you rotate the polarizer. If the polarizer's already off the camera, look through it, rotate it, and see for yourself if there's anything to be gained. You'll often be pleasantly surprised.
Scattered light, glare and other strong reflections can degrade photographs in many wayse.g., by diluting colors, by obscuring or distracting the viewer from important image details, or by forcing suboptimal exposure compromises. Luckily, nature tends to tag such "bad light" with varying degrees of polarization, and that marker provides an easy way to suppress the "bad light" while capturing the good.
Most of the primary light sources encountered in photographythe sun, moon, indoor lighting, flash lampsare unpolarized, meaning that the electric field fluctuations accompanying the light are oriented equally and randomly in all directions perpendicular to the light's direction of travel. If all the light's electric fields were oriented in the same direction, we'd say it's linearly or plane polarized. Other types of polarizatione.g., circular and ellipticalare seldom encountered by photographers outside their cameras.
Linear polarizing filters use optical materials with crystalline structures aligned in such a way that they readily pass incident light linearly polarized in one direction (we'll call it the passing axis) while strongly absorbing light polarized along the perpendicular direction, or blocking axis. These 2 optical axes always lie 90° apart.
Other polarizations are variably filtered out of the mix by absorption. Those closest to the blocking axis are attenuated the most.
For a beautifully illustrated and animated explanation of polarization, see this Polarization of Light tutorial.
Polarizing filters are not all created equally. For starters, there are 2 basic types, linear and circular, differing only with regard to camera compatibility and price. Linear and circular polarizers block polarized light equally well, but some cameras (SLRs in particular) don't work properly with linears, as detailed below. Circular polarizers are compatible all cameras but usually cost more than linears of comparable quality.
Polarizing materials also vary in their effectiveness and spectral properties. Construction quality enters the equation as well. My Tiffen circular polarizer is noticeably more effective at blocking polarized reflections than my Tiffen linear polarizer, but the performance difference reflects the quality of the materials in these two particular filters, not the circular vs. linear design. Polarizers normally impart a very subtle blue tint, but some are bluer than others.
Once you've established that your camera does or doesn't require a circular polarizer, blocking efficiency becomes the most important selection criterion. If you have the opportunity to compare several polarizers in person, test them all against a strongly polarized reflection like the one set up in the reflection calibration method outlined below and favor the polarizer that blocks the best. A multicoated polarizer will reduce the risk of flare, but when the situation calls for a polarizer, an uncoated polarizer will usually be far better than none at all.
The non-TTL techniques espoused in this article assume a polarizer with its blocking axis clearly marked as shown above, but some polarizers come with their passing axis marked instead, and many come with no dedicated markings at all—presumably on the assumption that they'll always be used with TTL control. At the other extreme, Heliopan's thin circular polarizer has a finely graduated rotating ring for precision work. My inexpensive Tiffen circular polarizer came with a single white dot sitting on its blocking axis, but I had to mark an even cheaper Tiffen linear polarizer myself.
Two simple calibration methods are discussed next.
This method uses a strongly polarized reflection as a reference.
Polarized sunglasses are always constructed such that the blocking axes of both lenses run right to left as the glasses are meant to be worn. You can use this fact to calibrate your polarizing filter:
Once you've determined your polarizer's blocking axis, mark either end of it on the outer rotating ring for future reference. (Ring positions 180° apart are functionally equivalent.) Some recommend a dot of nail polish for this purpose, but I've also had good luck with a sliver of white electrical tape. If your polarizer has writing on the outer ring, note any characters that happen to line up with the blocking axis.
In the field, I find it much simpler to work with a mark on blocking axis. Once I've discerned the polarization of the unwanted scatter or reflection to be suppressed, I can simply turn the outer rotating ring to align my mark with that target polarization.
Effective polarizer use involves careful attention to camera angle as well, but polarizer ring adjustment depends only on the orientation of the target polarization.
Light linearly polarized in a direction between the polarizer's passing and blocking axes is partially blocked. The passed intensity of such light varies with the square of the cosine of the angle ø between the polarizer's passing axis and the incident light's direction of polarization:
where I0 is the incident intensity and k is a constant between 0 and 1 representing the intensity fraction passed when ø = 0. (The better quality the polarizer, the more closely k approaches 1.)
Conversely, the percentage of linearly polarized incident light blocked by the polarizer is
where ø' is the angle between the polarizer's blocking axis and the incident light's direction of polarization. Note that ø and ø' are always complementary angles.
Atmospheric scattering occurs when gas molecules, water droplets and other airborne particles absorb and reradiate light, usually sunlight, in a process know as Rayleigh scattering. Scattering efficiency depends on particle size and the altitude (angular height) of the light source above the horizon, and varies inversely with the 4th power of the incident light's wavelength (color). Sunlight suffers the least scattering at local noon in clean, dry air, but the preferential scattering of shorter wavelengths slightly reddens the light reaching ground level even then.
As a process, scattering isn't all bad. After all, we owe our gorgeous blue skies and cherished red-yellow sunsets to it. Clean, dry air scatters blue light (~400 nm) about 7 times more efficiently than red (~650 nm) and about 3 times more efficiently than green (~520 nm). Violet (~350 nm) is actually the most strongly scattered color, but the human eye responds very poorly to violet, so blue sky is what we see. Late sunlight skimming the horizon loses much of its blue-green content to scatter over its long path through the lower atmosphere before ever reaching admiring eyes.
Adding moisture or particulates to the air substantially increases scattering efficiency at longer (redder) wavelengths. With more colors represented in their scattered light, clouds look white, and hazy (damp or smoggy) skies take on a grayish to whitish cast.
Luckily for photographers, details of the molecular-level reradiation process at the heart of scattering ensure that at all wavelengths,
whether it be sun, moon or searchlight. The polarization differentiates scatter from the desirable and largely unpolarized light reflected from our photographic subjects, and is thus our point of attack.
We're now ready to apply what we know about scatter to defeat it.
Note that at high noon, the arc of target directions will fall near or even below the horizon, depending on latitude, time of year and local topography.
Of course, you won't always be able to shoot in a target direction, but to the extent that you can, this polarizer technique will give you bluer skies, more conspicuous clouds and more vibrant colors throughout your scene, as you can see in the samples below.
Table Note: Automatic exposures; no post-processing, looking NW in late afternoon sun.
The vertically banded sky in the bottom left sample directly above nicely illustrates
To mitigate sky banding in wide angle shots,
BTW, bright sky suppression can also be had with graduated neutral density filters—sometimes with better results in wide angle situations near the ends of the day. But proper GND technique demands a nearly flat horizon and reliable TTL control—i.e., a clearly visible LCD image—and generally involves more setup and effort than polarizer technique. In my experience, GNDs make polarizers look easy, but they can be very rewarding at times.
Take your polarizer off the camera and observe the sky as your rotate it to confirm this technique with your own eyes. With a circular polarizer, you'll only see the effect if the male threads face you. With a linear polarizer, you'll see the effect either way, but if you align the rotating ring for a particular effect with the threads away and then flip the polarizer around to mount it, you might lose your hard-earned alignment.
Reflecting surfaces generally come in 3 textures: smooth, rough and in between. Reflections off smooth, mirror-like surfaces are termed specular reflections. By happy coincidence, specular reflections are the kind best controlled by polarizers and also the kind photographers most often need to suppress.
Smooth metallic surfaces generally preserve the polarization (or lack thereof) of the incident light on reflection. (Actually, such surfaces reverse the incident polarization, but with linearly polarized light, the reversal's not readily observable.) Since sunlight is unpolarized, sunlight reflected off a metallic surface will remain unpolarized.
By constrast, most smooth non-metallic surfacesmost notably water and glasstend to impart polarization on the light they reflect by absorbing or transmitting certain electric field orientations and reflecting (more accurately, reradiating) others.
A photographer armed with a polarizer can profitably exploit this fact:
Reflections from uniformly rough surfaces are termed diffuse. The even omnidirectional reflection off coming off baking soda neatly leveled in a bowl is diffuse. In between specular and diffuse reflections are spread reflections, which exhibit some directionality and may acquire some polarization.
For a beautifully illustrated and animated explanation of reflection, click here.
Just how much polarization specular reflections acquire in the reflection process depends on
Unpolarized light, the kind photographers usually encounter, can be thought of as a mix of components polarized parallel and perpendicular to the plane of incidence. A non-metallic specular reflector will reflect the incoming component polarized perpendicular to the plane of incidence (i.e., parallel or tangential to the reflecting surface) in toto, without change in either polarization or intensity. Meanwhile, a variable portion of the incoming component polarized parallel to the plane of incidence will refract (bend) into the reflecting surface, to be transmitted or absorbed as dictated by the optical properties of the reflector.
In other words, partial refraction effectively filters from the reflection some of the light polarized parallel to the plane of incidence, while the component perpendicular to the plane of incidence reflects unscathed.
At a certain angle of incidence known as Brewster's angle, the reflection ends up 100% polarized perpendicular to the plane of incidencei.e., parallel or tangential to the reflecting surfacebecause the component parallel to the plane of incidence has been completely refracted into the reflecting material.
For reflections in air (n = 1), the expression reduces to
The table below shows the Brewster's angles of most interest to photographers.
At other angles of incidence, the reflected light becomes only partially polarized, but the net polarization remains tangential to the reflecting surface. At 30° and wider angles (i.e., at 35 mm equivalent focal lengths under 85-100 mm), polarizer efficacy falls off progressively as peripheral reflections deviate more and more from Brewster's angle. Uneven suppression of reflections can be seen to some extent in the glare examples shown above.
Acknowledgement: Thanks to Chris Andreev for alerting me to polarizer coverage limitations as they relate to reflections.
Now we're ready to put all this reflection theory into practice.
their designers set the blocking axis of the polarizing coating horizontally across the frames. As you might have guessed, this arrangement does little to reduce atmospheric scatter, except along the horizon near local noon, but then scatter's not a major cause of squinting.
Of course, you won't always have the freedom to base camera angle solely on polarization, but knowing the ideal arrangement will allow you to exercise the freedom you do have to get the most out of your polarizer.
The table below illustrates the non-TTL reflection control technique in use.
Table Note: Automatic exposures; no post-processing.
Regarding the last item, you'll only see the effect with a circular polarizer if the male threads face you. With a linear polarizer, you'll see the effect either way, but if you align the rotating ring for a particular effect with the threads away and then flip the polarizer around to mount it, you risk losing your alignment.
In completely unpolarized light like sunlight or incandescent light, an ideal polarizer acts like a 1-stop (50%, 0.3 density) neutral density filter in any position. That said, real polarizers require more like 1-2 stops of exposure correction in unpolarized light, and even more in scenes with polarization. The exact amount will vary from one polarizer and one scene to another.
In unpolarized light on my C-2020Z, for instance, my Tiffen linear polarizer requires a 1 2/3-stop exposure correction (a filter factor of 3.0), while my Tiffen circular polarizer requires exactly 2 stops, or a filter factor of 4.0. I rather doubt that this slight difference in density can be generalized to other LP-CP pairs, even those from the same manufacturer.
Luckily, digital cameras with automatic exposure via TTL metering afford a number of ways to compensate for polarizer losses. Which one to use depends largely on which camera settings you need to control manually.
Many higher-end digital cameras now offer all these features.
Calibrating Your Polarizer For Manual Exposures
With a digital camera, it's fairly easy to calibrate a polarizer for manual exposure by analyzing a series of test images with PIE (Picture Information Extractor), a worth-its-weight-in-memory-cards image management utility that lists the exposure (EXIF header) data most digital cameras store within their image files.
If you prefer fully manual exposure control and use a polarizer a lot, you'll probably find this procedure worthwhile.
Circular polarizers (CPs) are 2-layer affairs—a linear polarizer (LP) backed with a special birefringent optical layer known as a quarter-wave plate (QWP). The QWP is bonded to the LP with its fast optical axis oriented 45° to the LP's passing axis. In this configuration, the QWP transforms the linearly polarized light emerging from the outer LP layer into circularly polarized light, which to the polarization-sensitive metering and auto-focus components found in many modern SLR cameras is indistinguishable from unpolarized light.
The QWP layer in CPs insures accurate TTL auto-focus and metering in all cameras but adds cost and another potential source of image degradation, as illustrated in the Fun with a Reversed Circular Polarizer section below.
Technical Note: For a beautifully illustrated interactive explanation of birefringence, click here.
You can usually go either way—CP or LP—with popular digital rangefinder cameras like the Oly C-20x0Z, C-30x0Z and C-4040Z lines, the Nikon CoolPix series and the Canon PowerShot G1 and G2. The straight-through designs found in such cameras usually don't involve polarization-sensitive internals like folded lightpaths and beam-splitters. However, digital SLRs like the Oly E-10 and E-20 and the Nikon D1 generally do, and they'll likely require a CP for proper metering and auto-focus. (The D1 definitely does, at least for metering.)
Generally speaking, LPs and CPs are equally effective at blocking polarized light. Oft-repeated claims that one typically outperforms the other have no basis in optics but have a lot to do with the variable quality of commercially available polarizers. Such myths also apparently feed off a widespread lack of appreciation for the optical complexity of the CP—particularly when reversed.
LPs are simpler, less expensive and theoretically optically cleaner than their circular counterparts. If you know for a fact that your camera meters and auto-focuses properly with an LP, feel free to go linear. Otherwise, play it safe with a CP.
Of course, things are never quite that simple. My particular Tiffen CP (~$25) blocks polarized reflections noticeably better than my Tiffen LP (~$15), presumably due to quality differences peculiar to the Tiffen line. (Near Brewster's angle, the LP passes a clearly visible reflection at maximum effect while the CP passes none.) For an extra $10, the Tiffen CP offers better performance and universal compatibility, too. Guess which one I carry?
I seriously doubt that the quality difference here has anything to do the CP vs. LP design. When LP compatibility isn't an issue, I'd choose a polarizer based on a balance of blocking performance and cost and let the LP vs. CP chip fall where it may.
It's easy to test your camera's LP compatibility, at least with regard to metering:
If your camera passes the LP metering test, an LP will in all likelihood be OK, but an LP-related auto-focus malfunction is still theoretically possible. I suppose you could test the camera's ability to auto-focus at several different polarizer positions over a 180° arc, but I have no reliable information here.
Conclusively establishing that a specific digital camera works properly with an LP under all circumstances can be challenging, as chronicled in The Great C-20x0Z Circular vs. Linear Polarizer Debate elsewhere on dpFWIW .
Polarizers aren't always clearly marked, but thanks to an RPD tip from Dennis Duke, it's easy to tell whether a polarizer in hand is circular or linear. All you need is a mirror:
How black the CP turns is probably an indication of its quality, particularly with regard to the spectral tuning of its QWP layer.
Dennis also notes that most polarizers not marked as circular are linear. That jibes with my limited experience with a CP and LP from Tiffen.
Fascinating, Captain, But How Does It Work?
Ready for a nice yawn?
In the table below, L represents the linear polarizing layer found in both LPs and CPs, Q represents the quarter-wave plate (QWP) layer added inboard to an LP to make a CP, and M represents the mirror. Thanks to physicist Joe Miller of RPD for helping me work this out.
Technical notes: 1. The explanations above reasonably assume unpolarized ambient light. 2. Some vision physiology sources claim that the human eye can detect and the brain can perceive polarization, but the effect is at best extremely subtle. 3. Birefringent materials like cellophane transmit light faster in some directions than in others due to anisotropies in their molecular structures. The directions of maximum and minimum light velocity (or equivalently, of minimum and maximum refractive index) are known respectively as the fast and slow axes of the material and are generally 90° apart. The difference in light velocity (or refractive index) between the fast and slow axes determines how thick a given birefringent material must be to function as a quarter-wave or half-wave plate at a given wavelength.
Look at a polarized light source through your CP with the male threads pointing away from you, and you'll see some very interesting color shifts that nicely illustrate several important polarization concepts, thanks to the surprisingly complex optical properties of a typical QWP. Henceforth I'll refer to a CP held male threads away as a reversed CP.
It might just be possible to mount a reversed Cokin-style CP to put this effect to use, but that's pure speculation on my part.
The Experimental Setup
To explore reversed CP behavior, I created a simple desktop scene containing a highly polarized reflection provided by a cup of still water positioned to reflect the bulb of a desk lamp near Brewster's angle, ~55° off the normal to the water's surface. (An uncoated filter would have made an equally good reflector, and the optimum angle would have been about the same.)
The reflection coming off the water was highly polarized in a direction both tangential to the water's surface and perpendicular to the eye-water line. Light coming from the scene around the water was largely unpolarized, including light coming directly from the bulb.
Viewing this scene (water + surroundings) through a rotating reversed circular polarizer (CP), I consistently observed orientation-dependent color shifts, but only in the water reflection—i.e., only in incident light bearing a net linear polarization. No color shifts ever appeared in the unpolarized light coming from the scene around the cup.
The white dot on my Tiffen CP sits on the LP layer's blocking axis. As expected, the unreversed CP maximally blocked the water reflection with white dots at 3:00 and 9:00 (i.e., parallel to the surface of the water) with no color shifting whatsoever.
With the CP reversed, however, I saw just the opposite—mild but noticeable color shifting limited to the water reflection but little if any overall reflection blocking. White dots at 3:00 and 9:00 always yielded the bluest color shift, while white dots at 12:00 and 6:00 (90° to the water's surface) always gave the reddest. White dots ~45° between these cardinal positions seemed to yield a neutral tone.
What's It All Mean, Mr. Natural?
As noted briefly above, a CP consists of 2 layers: A simple LP on the outside and a birefringent QWP on the male-threaded side. The QWP has a molecular structure that transmits light at different speeds in different directions. The directions of greatest and least speed are known respectively as the QWP's fast and slow optic axes. In QWPs, as in most birefringent materials, these optic axes are 90° apart.
On exiting an ideal QWP, the phase of incident light linearly polarized along the QWP's slow optic axis is retarded by exactly one quarter wavelength relative to incident light polarized along its fast optic axis. When the QWP is oriented with its optic axes offset 45° from the pass direction of the LP layer in a CP, this phase shift "remixes" the linearly polarized light emerging from the LP into circularly polarized light. To polarization-sensitive camera internals, circularly polarized light is indistinguishable from unpolarized light. All CPs are constructed with this 45° LP-QWP offset.
Real-world QWPs can achieve perfect linear-to-circular polarization conversions only for a relatively narrow band of wavelengths. All other wavelengths in the incident linearly polarized light exit the QWP with an elliptical polarization that acts like a residual linear polarization. (Linear and circular polarizations are in fact opposite extreme cases of elliptical polarization.) To spread the misery across the visible spectrum with minimal impact, the QWPs used in CPs are usually tuned to green light (~520 nm) at the peak of the solar power spectrum.
The green wavelengths in the incident polarized light thus emerge from the CP with perfect circular polarization, while the blue and red components come out with elliptical polarizations. The long axis of the red polarization ellipse remains roughly aligned with the incident plane of polarization, but the long axis of the blue polarization ellipse deviates by nearly 90°, as we'll see below.
QWP In Action
With the CP reversed, light from the scene first hits the QWP, which does no filtering itself. Aside from minor reflections off its incident face, the QWP passes all the light it receives, polarized or not.
The QWP has no effect whatsoever on the unpolarized light coming off the scene. Nor does it have any effect on light polarized along its fast or slow optic axes, as occurs in the setup above with the CP's white dot at 1:30, 4:30, 7:30 or 10:30—hence the neutral coloration observed at those positions.
Interesting things happen, however, when the reflection is polarized at 45° to the QWP's fast axis, as occurs in the setup above with the CP's white dot at 12:00, 3:00, 6:00 or 9:00. In these cases, the QWP does its usual thing—it converts the linearly polarized reflection into a circularly polarized green component and elliptically polarized red and blue components of varying orientations.
The subsequent LP layer indiscriminately blocks exactly half the unpolarized light and exactly half the circularly polarized green light coming through the QWP. However, it blocks more or less than half the elliptically polarized red and blue light, as we'll see next.
The reversed CP color shifts observed at 12:00, 3:00, 6:00 or 9:00 arise here, but why maximum bluing at 3:00 and 9:00 and maximum reddening at 12:00 and 6:00 white dot positions? To understand that, we'll need to take an even closer look at the spectral properties of the green-tuned QWP.
The Colors, Man, the Colors!
At short blue (~450 nm) and violet (~400 nm) wavelengths, a QWP tuned for green light (~520 nm) begins to look and act more like a half-wave plate (HWP) than an ideal QWP. The HWP is another curious birefringent optical device with the ability to flip the polarization of incident linearly polarized light by exactly twice the angle between the HWP's fast axis and the incident plane of polarization.
Now, at 3:00 or 9:00 white dot positions, the reflection's incident polarization is squarely aligned with the LP layer's blocking axis but 45° off the fast QWP axis. The resulting blue-violet polarization flip of 2 x 45° = 90° swings the long axis of the reflection's blue-violet polarization ellipse into rough alignment with the LP layer's passing axis instead. This allows some of the blue-violet light to sneak through the LP layer.
The longer red (~650-700 nm) wavelengths don't fare as well, however. A green-tuned QWP has relatively little effect on the polarization of red light within the water reflection. At 3:00 or 9:00 white dot positions, the elliptically polarized red component emerging from the QWP, still roughly aligned with the LP layer's blocking axis, suffers heavy attenuation. Thus red gets filtered out disproportionately.
On balance, the LP layer ends up being more transparent to blue-violet than to red light at the 3:00 and 9:00 white dot positions, thanks to the spectral properties of the reversed CP's imperfect QWP layer. The result is maximum bluing at 3:00 and 9:00, just as I had observed.
The same mechanism nicely accounts for the observed maximum reddening at 12:00 and 6:00. I'll leave it to the interested reader to work through the details for those positions.
Anyone still awake? Didn't think so.
Why fool with polarizers at all when you can clean up digital pictures in post-processing?
Well, for one thing, you'll have a mighty hard time editing out interfering reflections in shots taken through or around glass or water. You'll also have a heck of a time editing out white-out reflections from foliage in bright sunlight.
You might have more luck with atmospheric scatter. For example, you could shoot the exact same bright-sky scene twice, once with spot-metering in the foreground and again with spot-metering on the sky. Or you could just crank down 3 stops on the sky shot. You could then cut and paste the two together in post-processing and airbrush, blur or otherwise obscure the seam. Just pray there aren't any trees, buildings or other pesky objects (subjects?) around to bugger up your skyline.
At bottom, you have to ask yourself, just how good are you and your software at post-processing, and how much time do you have to spend on it?
Personally, I'd rather rely on a polarizer.
Sorry for this long-winded "more than you ever wanted to know about polarizers" exposition. I just couldn't resist. Something about polarization physics tickles me, strange as that may be.
(See also the home page links.)
Hunter, Fil and Fuqua, Paul, Light — Science & Magic, Focal Press, 1997.
Rowell, Galen, Mountain Light, 2nd ed., Yolla Bolly Press, Sierra Club Books, San Francisco, 1995.
Filters for Outdoor Photography—an excellent introduction to polarizing, GND and other filters by accomplished photographer Darwin Wiggett.
Feynman, RP, Leighton, RB, Sands, M, The Feynman Lectures on Physics, Vol. 1, Addison-Wesley Publishing Company, Reading, MA 1963.
Encarta 99 Reference Suite, Microsoft, 1998.
Jens Roesner's polarizer article—still under development but already with brimming graphs and demonstration photos on polarization. Jens also discusses the limited blocking of near IR by crossed linear polarizers.
UV and Polarizing Filter FAQsa well-written and informative FAQ section within a site that otherwise appears to be largely under construction.
"Tiffen Filter Facts"—a well-illustrated "infomercial" pamphlet packaged with Tiffen filters. See also the Tiffen filter web page has filter specifications and many color illustrations of filter effects.
Paul Saunders, Wilderness Wales.
Unless explicitly attributed to another contributor, all content on this site © Jeremy McCreary