Get clued-up on how CPL filters work.
A circular polarising lens filter, also known as a CPL filter, is an essential bit of kit for any landscape or outdoor photographer. Understanding how a CPL filter works is useful to maximise the potential of this type of filter, and the different effects it can achieve.
WHAT IS A CPL FILTER?
A CPL filter is a screw-in, polarising filter that reduces or controls the amount of light that reaches your camera lens. The filter is manually rotated one way or another to varying degrees, to influence exposure to light levels.
Adjusting light exposure reduces reflections, glare and haze. This also boosts colour saturation and contrast, making images appear richer, clearer and more vivid.
HOW A CPL FILTER WORKS
But, just how does a CPL filter work to achieve such positive results?
Visible light from the sun moves in a straight line but acts like waves, moving in all directions and angles. When this unpolarised light is reflected off something, the object’s colour is determined by the reflected wavelength of that light. Where reflected light travels in a single, polarised direction, it creates glare and reduces colour intensity of a reflected surface.
A CPL filter is made from a wave-retardation plate, a quarter of a wavelength in thickness. It can be rotated to block certain wave movement directions, to help remove polarised light. This restores colour intensity and eliminates glare and reflections. The amount of polarised light that’s filtered depends on how much the filter is rotated and the angle degree.
Adjusting a camera’s line of sight relative to the sun influences the polarising strength. The maximum polarisation effect takes place at an angle of 90 degrees from the sun or source of light. To understand where to position your camera to achieve this effect, use this tried-and-tested trick: Make an L shape with your thumb and index finger, where your thumb should point towards the sun. The index finger should dictate the angle you need to shoot at. Some photographers also wear polarised sunglasses to predict how their images will appear.
By finding the optimum angle to filter diffused light, you can boost colour saturation, remove surface reflections and achieve greater contrast. For example, the Urth 77mm CPL 3Peak blocks sunlight directly reflected towards the camera, so makes for a good choice of outdoor photography lens filter.
Since the angle influences the polarisation effect, it’s common to get uneven results when using a CPL filter with a wide angle lens. This is because some aspects of the picture are in line with the sun, while others aren’t, thus creating varying degrees of polarisation strength in a single scene. Visible darkening in the corner of an image, or vignetting, may also occur using a CPL filter on a wide angle lens. You can also reduce vignetting by not stacking CPL filters with other types.
Another type of polarising filter is the linear filter. CPL filters are the more common of the two types. They fit onto virtually any camera lens and function effectively on digital cameras. On the other hand, linear filters can hinder camera metering or auto focusing accuracy. So, these are only suitable for use on a manual focus lens. Essentially, a CPL filter is a linear filter with an extra layer of glass that polarises light in a circular motion. This creates exposure accuracy when light hits the camera’s metering system.
Understanding how a CPL filter works largely depends on experimentation. Play around with the filter to see what different effects it can achieve. However, just because you can rotate your filter to achieve the full polarising effect, doesn’t always mean that you should. This is especially true if you want to retain a natural-looking feel to your photos.
When light is filtered from a camera, light exposure is reduced by around one or two stops. To compensate for this, slow down the shutter speed, open up the aperture slightly or increase the ISO.
A CPL filter will work to its maximum efficiency if you use a high-quality filter, preferably with a multi-resistant coating. Make sure that you get the right diameter size for your particular lens.