Circular Vs Linear Polarising Filters

Polarisers are one of the most misunderstood filters on the market and photographers often question the difference between the linear and circular versions. 

There is a common misconception that 'circular' refers to the shape of the filter. This is in fact not the case as both circular and linear polarising filters can look exactly the same. Here we aim to give you a better understanding of polarising filters and what they do.

What is Polarization?

Before we delve into the differences between circular and linear, it's best to gain a better understanding of what polarising itself is. 

Basically, polarisation refers to the behaviour of light waves that are hitting your lens. Light waves travel in different ways and the easiest way to visualise this is to take a physical example. By taking a piece of string and attaching it at one end to something you can replicate waves. Move the string up and down and the light wave produced is vertically polarised. Move the string from side to side and the wave is horizontally polarised. These two are referred to as linear polarisation. Spinning the string around in a circle can lead to either right-handed or left-handed polarisation. this is referred to as circular polarisation. 

The term polarisation basically means that light is oriented in one direction, making it predictable and not random. 

Linear Polarizing Filters 

Linear polarizers are comprised of two elements that can be twisted to alter the direction of the light waves that are allowed through. So at one angle it will allow the passage of horizontal waves but when rotated by 90 degrees would only allow vertical waves. 

This is ideal when wanting to suppress one direction of light more than the others. In reflections, especially from water and partially reflective surfaces such as windows there will be one direction of light that is stronger than the others and by suppressing that, you'll lessen the amount of reflection vastly. 

Polarizers are also known for darkening the sky by blocking light rays from the sun or certain polarisation and saturating colours by lessening the reflection of less reflective things like leaves. Atmospheric haze is also caused by scattered light and so a polarizer can help to lessen this too. 

Circular Polarising Filters

Circular polarisers are a little more complicated to explain. They are sensitive to both linear polarisation and the left and right-handed circular polarisation. A circular polarizer consists of two elements; a linear polarizer just like above, and a quarter wave plate which is stuck to the back of the linear polarizer with a specific orientation so that the light emerging from the quarter wave plate is circularly polarised. This is where the name comes from. 

So, why do we need circular polarizers if the provide the same photographic results that a linear polariser would? Well, it's all down to the camera kit you have. If you have a DSLR, then the likelihood is that it has a partially reflecting mirror. Because of this partial reflection, metering errors can occur when using linear polarizers with DSLRs that have partially reflecting mirrors. Using a circular polarizer can eliminate these issues. 

Even if your camera doesn't need a circular polarizer, you can still use on and it won't make a difference to how it affects the image. 

Hoya, Cokin and Kenko all produce circular polarizers in a variety of thread sizes. Take a look at the 'general filters' section of filterzone to see them in action.