The Science of Polarizing Filters

While Photoshop and its many imitators have rendered a lot of camera gear useless, there is still one physical filter which Photoshop cannot emulate – the polarizing filter. The systematic screening of certain kinds of light is a job that can only be done during capture. While polarizing filters are among the most useful tools for outdoor photographers, they also seem to be the least understood.  ”Stick it on and rotate it ’til it looks right” seems to be the most popular technique.  In this article, we will cut through the black magic and determine how polarizing filters work and what the best techniques are.

Using a Polarizing Filter on the Sky

One of the most popular uses of a polarizing filter is to darken the sky in a landscape portrait.  The deep blue tones that are uncovered with a polarizing filter will definitely add some interest to your landscape photos.  The science behind this technique begins with sunlight impinging a water molecule in the atmosphere.  Light from the Sun is originally unpolarized – that is, the electric fields within the light waves vibrate in all directions.  We’ll represent this in the diagram below by showing the light with both horizontal and vertical components. Once the light impinges the water molecule, the electric field within the light induces a vibration within the water.  Light is then reradiated from the water molecule as polarized light at a right angle to the incident angle.  If this region of the sky is then viewed with a polarizing filter, a great portion of the light can then be filtered out by rotating the filter until its axis is oriented oppositely to the polarized light.  (Hint:  There is normally no way to know the axis orientation of your polarizing filter, so just rotate the filter until you observe its maximum effect.)

The right angle polarization of sunlight through the atmosphere is what causes different regions of the sky to look differently when viewed through a polarizing filter.  Regions of the sky which form perfect right angles between the viewer and Sun will appear as dark, deep blues.  Regions at other angles will be increasingly unaffected by the polarizing filter.

You can predict areas of maximum effect for polarizing filters by pointing your index finger at the sun with your thumb extended upward in “gun” fashion.  Then rotate your hand while your index finger continues to point at the Sun.  Anywhere your thumb points will be an area of maximum effect for a polarizing filter.

The figure below illustrates the increasing effect of a polarizing filter as the angle from the Sun increases. The strong variance in polarizing filter effect is also why many people recommend against polarizing filters for wide-angle lenses.  For wide fields of view, sharp differences in polarization create vastly different shades of sky across the photo.  It is really up to you and your artistry to make variances in sky work in your photograph.

Using a Polarizing Filter to Decrease Water Glare

Another popular use for a polarizing filter is to decrease the amount of glare in water photos.  Unlike the 90 degree rule for filtering light from the atmosphere, we must consider a slightly different set of rules for filtering light reflections from large surfaces of water.  For light reflections off of a medium, the angle of maximum effect for a polarizing filter is dictated by Brewster’s angle.  This angle is dependent upon the properties of the two media which form the interface – in the case, air and water.  The figure below shows that when sunlight impinges a body of water at just the right angle (Brewster’s angle), the reflected light is almost perfectly polarized.  If we then orient the axis of our polarizing filter opposite the polarization of the reflected ray, the result is a perfectly transparent water interface (i.e. no glare).

For an air-water interface, Brewster’s angle turns out to be 53 degrees.   Note that this angle is measured to an imaginary line perpendicular to the water surface.  The Sun-water angle (and filter-water angle) would be 37 degrees for maximum effect. The graph below shows what happens when you try to photograph at angles other than Brewster’s angle.  Think of the red line as the polarization component we are trying to eliminate.  Then the blue line will be the component we filter with our polarizing filter.  At any angle other than 53 degrees, there will be some red component (and consequently a glare on the water).  At 53 degrees, there is no red component and with a properly oriented polarizing filter to screen out the blue component, you will have crystal clear water.

Using a Polarizing Filter to Decrease Glare from Glass

The Brewster’s angle concept also holds true for an air-glass interface.  If you are trying to photograph through a glass window and are having trouble with glare, try using a polarizing filter and use Brewster’s angle to your advantage.  In the case of an air-glass interface, Brewster’s angle is 56 degrees.  This means if your camera is pointed at a 34 degree angle to the glass, you should be able to tune out all the glare with your polarizing filter.

Conclusions

A polarizing filter can be one of your best tools for taming harsh light. Try out some the techniques above when you get a chance and you might be surprised how much farther you can take your photography when you have science on your side.

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