Polarising filters should be one of the first tools in the camera bag for an outdoor photographer, as they suppress reflections and reduce the effect of atmospheric haze. Landscape photographers often fit polarisers to darken the sky and make white clouds more prominent. And they are also handy when you want to subdue glare from reflective surfaces like water and glass.
The image on the left was shot without a polarising filter while the one on the right was captured through a polarising filter and shows how it can darken blue skies, suppress reflections and boost contrast and colour saturation.
Polariser Types
There are two types of polarising filters: linear and circular. Linear polarisers can create problems with the autofocusing and light-metering sensors in some cameras. Circular polarisers combine a linear polariser with a quarter-wave plate which converts the resulting linearly polarised light into circularly-polarised light before it enters the lens.
Unpolarised light passing through a linear polariser enters the quarter-wave plate at a 45-degree angle. When this light leaves the quarter-wave plate it is rotated exactly one quarter of a wavelength and emerges circularly polarised. (Source: http://commons.wikimedia.org/wiki/User:Dave3457.)
Both types of polariser can achieve the same objectives with respect to their effect on light. However, most digital camera users will require a circular polariser to minimise any chances of interference with autofocusing and metering.
Traditionally, all polarising filters were circular disks that attached to the filter thread at the front of a lens. Screw-in polarisers contain two plates of optical material (not necessarily glass); one fixed in place and the other rotating.
Turning the rotating element changes the image brightness and reflectivity of subject within the scene. You may also notice some changes to the intensity of colours.
System filters
French manufacturer Cokin was the originator of the holder-based Creative Filter System, which was launched in 1978 and initially concentrated on colour effects and graduated filters. The system was originally designed with square filters that fit onto the lens with removable adaptor rings. Different lenses can be accommodated by using different size adaptor rings.
Today, Cokin’s filters are made from CR-39 organic glass, a plastic polymer commonly used in prescription glasses because it is transparent in the visible spectrum and boasts the highest abrasion/scratch resistance of any uncoated optical plastic.
Cokin’s system includes both linear and circular polarisers. The linear polarisers are single square CR-39 sheets in four sizes. The circular polarisers also come in four sizes but are themselves circular in shape. Unlike other resin Cokin filters they are mounted in glass and fit into the filter holder’s back slot so they are easy to rotate.
Cokin’s circular polarisers are round in shape and designed to fit into the system’s holders.
Using Polarisers
Polarisers work by filtering out light that has been directly reflected toward the camera at specific angles. The angle that is filtered is controlled by rotating the front element of the polariser, while the strength of polarisation is controlled by changing the camera’s line of sight relative to the sun.
The maximum polarisation occurs when the sun is at 90 degrees to the subject. A good way to visualise this is to aim your index finger at the sun (like a gun) while holding your thumb straight up.
Some fine-tuning will probably be required to establish where the filter will have its greatest effect. Rotating the front element of the filter while viewing the scene through the camera’s viewfinder or rear LCD allows you to choose the degree of polarisation that best suits the subject and your objectives.
When direct reflections are reduced, a polariser will also increase colour saturation because it enables more of the light from the subject to travel directly to the sensor, instead of being diffused as it is reflected from shiny surfaces. The intensity of blue skies will increase and foliage will have a more intense green. Sometimes the end results are attractive but often they make images appear unrealistic.
Polarisers have their greatest effect on clear sunny days, when it is important not to overdo the effect and create images with unusually dark midday skies or overly vibrant foliage. In overcast conditions and on wet days, you may struggle to achieve much in the way of suppressed reflections.
Correctly used, a polariser can increase the dynamic range in an image and make it easier to record scenes containing a bright sky with relatively unreflective land. This is achieved by decreasing the glare from reflective surfaces while darkening the sky.
In general, using a polariser for shots of scenes containing clouds and skies will almost always increase contrast. However, if the subject itself is highly reflective then a polariser is more likely to decrease contrast.
Because polarisers are highly angle-dependent, they are best used with normal and telephoto lenses. Wide angle lenses (28mm equivalent and wider) can produce uneven results. Parts of the scene that face directly into the sun will be strongly polarised, while those towards the side of the frame will not.
With wide angle lenses, rotating the polarising filter until the most pronounced polarisation coincides with the image’s edge or corner produces a more natural-looking result. The resulting change in gradient looks more like the natural gradient that occurs at twilight.
Use a polariser judiciously, and remember that removing glare can make a scene look very unnatural, and can also make water look great, or fake.
The panorama shown here was stitched together from six images shot with a polarising filter on a 24mm wide angle lens. Note the tonal variations in the sky, which reflect changes in the degree of polarisation as the camera’s line of sight relative to the sun is changed.
When you want to use a polariser to suppress reflections, the camera should be at an angle of around 35 degrees to the reflective surface. In this role, it can allow the photographer to choose between subjects which are reflected from or are underneath the surface of a sheet of water or remove unwanted reflections when shooting through a window or other transparent barrier.
However, a polariser can produce unwanted intensity and colour patterns when photographs are taken through windows that have been tinted or treated with coatings. A good example is aircraft windows which are laminated with plastic that can split the light into different colours. Slight variations in the relative position of the plastic layer can create colour or tonal patterns on the photograph.
Disadvantages
While polarisers are often very useful, they have a number of disadvantages:
1. They normally reduce the amount of light reaching the image sensor by between two and three f-stops. Exposures must be increased to compensate by increasing the ISO, opening the lens aperture or choosing a slower shutter speed (or a combination of two or more strategies). Cameras with TTL metering will adjust the exposure according to their in-built programs.
2. They have potential to reduce image quality if they are made from inferior materials and are not kept perfectly clean.
3. They require greater care with camera positioning and it can take longer to compose shots as the filter has to be rotated to the appropriate position.
4. It’s almost impossible to use a polariser successfully with stitched panoramas due to variations in the intensity of the polarisation as the angle of the camera to the sun is changed.
5. While a polariser can sometimes enhance the colour and contrast of a rainbow by darkening background clouds, it can be tricky setting it to the optimal rotation. It may be difficult to record the ends of the rainbow with a polariser because of uneven polarisation when using a wide angle lens.
6. There are times when the reflection makes the picture. For these shots a polariser is undesirable.
Avoid using polarisers when you want to photograph reflections.
Article by Margaret Brown – see Margaret’s photography pocket guides
