Saturday, July 9, 2011

Filters: Using a polarizer

The ability to alter images in post-processing of digital photography has eliminated the need for most of the enhancement filters that had been so popular in film photography.  In fact, the digital sensor is so much more sensitive to optical quality that placing any filter -- especially one of poor optical quality -- in the optical path will most likely be detrimental to the quality of the final image.  One filter effect that does not lend itself to complete obsolescence in the digital age is the polarizer -- although one of the more common uses of the polarizer, darkening blue skies, can usually be done with simple post-processing.

A popular use of a polarizing filter is to darken a blue sky.  With modern digital photography and post-processing this is not as important a use as it once was.  In the photo above a polarizer was not necessary.  The sky was simply darkened and contrast added afterwards in Photoshop.

What to look for:

Polarizing filters come in two varieties: linear and circular.  On a camera incorporating either AF (auto-focus) or built-in metering -- which is to say all modern DSLR cameras -- use a circular polarizer.  A linear polarizer will disrupt the AF ability of the camera and give false readings to the built-in light meter. 

A polarizing filter must turn to produce its result.  This necessitates a filter mount that may be thicker than an ordinary filter, and can cause vignetting with wide angle lenses.  Better quality polarizing filters are specially made with thinner mounts.  As with any filter used in digital photography, it is best to spend a little extra for a higher grade filter mount, optical glass, and coating of the glass.  This will have the least negative impact on the optical system of the camera.

Darkening blue skies:

A polarizing filter can be used to add punch to a landscape or travel shot by intensifying a blue sky, particularly if there are interesting cloud formations present.  The clouds will stand out more dramatically against the deeper blue.
The photo of the Empire State Building on the left is without a polarizer; that on the right is with.  Notice how the sky is darkened, plus additional color and contrast are added to the building in the polarized image.
 As already mentioned, much of this darkening can also be done in post-processing of the image.  Nonetheless, when you combine the ability of the filter to also add deeper color to other reflective surfaces in your shot, using the polarizer may be a better solution that post-processing.  Keep in mind that a polarizer will only work on a blue sky.  It does nothing for a cloudy sky. The ability of a polarizing filter to darken a sky depends on the type of sky and your shooting angle in relation to the sun.
 
A polarizing filter has its greatest darkening impact on a sky when the section of the sky being photographed is at a 90 degree to the sun, as in the illustration above.  At sunset, with the sun directly behind you at 180 degrees a polarizer will have no effect whatsoever
The polarizer works best when it is used closest to a right angle (90 degrees) from the sun.  With the sun behind you or overhead, turn the polarizer in its mount until you see the darkening effect in the camera viewfinder.  A polarizer can darken the exposure by as much as 2-stops.  Be aware of this and make any necessary exposure adjustments in your cameras, if you are in manual exposure mode.  Your camera meter should be able to compensate for this adjustment automatically.  

This image illustrates two potential problems with adding a polarizer to a wide angle lens.  First, the filter mount was too thick for the 28mm wide angle lens on this full-frame camera.  This resulted in dark, vignetted corners in the image.  Second, the sky varies in darkness from left to right resulting in uneven exposure.    This is amplified more through use of the polarizer.  Here is an instance where no polarizer would have been better.

Using the polarizer in conjunction with a wide angle lens may cause other problems.  The blue sky changes intensity as it moves from the 90 degree angle in relation to the sun.  A polarizer can negatively emphasize this exposure contrast when using very wide angle lenses.  

A second thing to look for with a polarizer on a wide angle lens is vignetting, which is the darkening corners in the image.  This is caused by the lens actually seeing part of the filter mount and recording it as a dark out-of-focus area.  Be aware of these problems.  You may not actually be able to see them occurring through the viewfinder.  When using an extreme wide angle lens, it is best to also take a backup shot without the filter in place.

Restoring color to foliage and other reflective surfaces:

One of the most useful implementations of a polarizing filter is in landscape photography for bringing out the saturated color of foliage.  A polarizer also cuts down the exposure, sometimes by as much as one or two stops.  This can be an aid to photographing moving water where you want to create a blur of the water by using a slow shutter speed.
Without a polarizer to cut the reflections the leaves in the top photo are completely washed out through over-exposure.  In the bottom image the full color and saturation of the leaves is restored by turning the filter to eliminate the unwanted reflections.
A typical example of where a polarizing filter is important.  It was used in conjunction with a neutral density filter to lower the exposure to 1/2 second which added a blur motion to the moving water.  At the same time the polarizer cut out the reflections on the foreground leaves and rock surfaces to result in a deeply saturated photo with plenty of color and detail.
One of the most important reasons to use a polarizer is so you can restore color to reflective surfaces.  This is particularly prevalent when photographing landscapes with foliage.  The surface of leaves are pointed skyward.  Since the leaf surface is glossy, it will reflect the pure light falling on it from the sky.  This will result in loss of color and actual over-exposure of the bright, reflective surface on the leaf.  A polarizer can eliminate this reflection and restore the natural color beneath it.  

This is a dramatic example of how a polarizing filter can eliminate the sky reflections in foliage and bring out the true colors of the leaves and flowers.
Here the polarizer is doing double duty by darkening the blue sky and also bringing out the colors in the autumn leaves.

Eliminating unwanted reflections:

The ability to remove unwanted reflections can be beneficial when photographing any reflective surface, such as glass, metal, water surfaces, or even shiny skin. 
The polarizer works best at a 90 degree angle to what is being reflected.  The photo on the left of a store window reflects the street scene to such a degree that the sign in the window is almost illegible.  On the right the polarizer almost completely eliminates the distracting reflection and now the sign behind the window is clear
Use of the polarizer on the right image eliminated the white sky reflections in the statue and brought out more color in the metal.
The only difference between these two images is that the polarizer was used for the photo on the right.  Color saturation is restored, glare is eliminated.
 The polarizing effect can sometimes be overdone.  It is not always necessary to turn the polarizer to its full effectiveness.  This may eliminate reflections that are integral to the final photo.
In this instance, the polarizing filter totally eliminated the reflection of the sailboat in the water.  Including some degree of the reflection would have made the shot more interesting.
Here the polarizing filter was only partially turned -- just enough to deepen the colors in the water and the shiny skin of the model, but not so much that it eliminated the important reflection of the girl in the sand.
As always, proper use of an aid to photography should enhance the creative outcome of the image, not detract from it.  Know the limitations and abilities of your equipment and accessories.  Sometimes knowing when not to use a piece of equipment is as important as knowing when to use it.


All content copyright 2011, Tom Grill Images, LLC

1 comment:

  1. Great post Tom, thanks a lot. I bought a polarizing filter along with my first DSLR camera in 2007, but never really understood why the shopkeeper had recommended it to me until about a year ago (when I started realizing its potential for blue skies). But your advice for any shiny surface is great! I never would've thought to use it like that! I'll have to go out on a photo shoot to explore its capabilities.

    I also couldn't figure out why I was getting vignetting...Thanks :)

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