A circular polarizer filter (CPL filter for short) is a landscape photographer‘s best friend. Calling it an accessory is almost reductive.
You should have it mounted on your camera lens for practically every shot you take.
In this article we’ll take you through everything you need to know about CPLs. From what they are to how to use a CPL filter to achieve results you can’t create through post-processing.
How Does a CPL Filter Work
The first thing to do when talking about optical elements to add to your photo setup is to understand how they work.
Each additional element brings with it some advantages but also adds complications to the optical scheme. This is because it affects the light rays that reach your digital camera’s sensor.
Understanding the theory behind the polarising filter will help you get the most out of it.
Types of Light Polarisation
In nature the light can be polarised in a linear or circular way.
Linear polarisation happens when you have light reflections in water, some types of plastic, glass, or when it goes through through the atmosphere under particular angles.
Circular polarisation is typical of reflections on metallic surfaces. That is when the light is reflected by conductive materials.
The circular polarizing filter then imparts a cyclic variation of the orientation of the electric and magnetic forces to the non-polarised light, selecting the appropriate component. It too appears dark and absorbs light in front of the camera lens.
The effect in photography of these two types of filter is identical. The difference between the two lies in the fact that the CPL filter first polarises the light by selecting a preferential polarisation plane. Then it depolarises it again.
CPL Filters and Modern Cameras
A CPL filter is the only one you can use with autofocus cameras. The phase-detection automatic focusing systems can’t operate in the presence of linear polarised light.
By using two polarisers, with the polarisation plane oriented at 90° to each other, you obtain almost complete darkness. This is because the light polarised by the first is barred by the second.
There is also a mathematical formula, which describes the dampeniing of light intensity through a polarising filter. It is called the Malus formula.
When to Use a CPL Filter and How
A CPL can help you increase your image quality, whether you’re photographing in a national park or on a travel assignment in India.
The Malus formula indicates how much the light intensity that hits the sensor will be dampened once it crosses a CPL.
For example, let’s say the angle between the input direction of the light beam and the output direction of the same beam through the CPL filter is zero degrees. The light will completely cross the filter, i.e. it will not be minimally affected. You won’t have any polarisation.
If the same angle of entry and exit is 90° then the two directions form a right angle between them. We will have the maximum absorption of the light beam and also maximum polarisation.
With the way CPLs are build, you have a rotating ring that allows you to move the filter. You can orient the plane of the polarisation, until you get the desired effect.
What to Do About the Sun
The Malus formula tells us that the best polarisation occurs when the incoming rays in the CPL are at 90° with respect to the rays coming out of the polarities.
This means that you will have the maximum effect of the polarisation when the sun is to the side.
With the sun in front of or behind you, the effect of polarisation will be minimal or even nil.
What Are the Effects of a CPL Filter
As a first and most important effect, the polariser helps you eliminate reflections.
A linear polariser will eliminate reflections from the water surface, some types of plastic and glass, and other non-metallic surfaces.
A circular polarizer, on the other hand, will eliminate reflection from metal surfaces as well.
Using a CPL filter will return transparency to a body of water, eliminating the reflection, and giving you high-quality images. You’ll be able to see the bottom of a sea or lake, for example.
Wood and metal paints polarise the light so you should use a polarising filter. Protective lacquers do the same.
A CPL is great on any time of glass too. It will eliminate the reflection, making the objects behind the glass visible. Think of paintings or exhibits, for example.
The CPL filter is not all powerful though. Glass is difficult to photograph due to the many reflections, so a leftover reflective residue might remain.
Darkens the Sky
Another classic effect of the CPL is to darken and saturate the blue sky. It will result in an intense blue sky with a gradient sloping towards the horizon line.
You can see this effect on the sky on clear days with strong contrasts given by the clouds.
As seen before, the effect is greater by placing the sun to the side of the shooting point. You will attenuate the effect if the sun is at your back.
Even when shooting against the light, the effect of the CPL filter is completely absent.
During sunrise and sunset or on very cloudy days it is very difficult or even impossible to darken the sky and polarise the light.
In these conditions I use the CPL filter only to remove reflections the surfaces.
Last consideration on colours. A CPL filter can saturate colours. Removing the reflections and filtering the light that enters the lens affects the sensor of the camera, leading to this effect.
It does so with deep blue skies, but also by intensifying the green colour of foliage.
In addition, it removes the white glow of incandescent light from leaves, making them a pleasant uniform green colour.
When Not to Use a CPL Filter
When There’s Not Enough Light
The first aspect to keep in mind is that the polarising filter reduces the amount of light entering the lens and affecting the sensor of the camera.
For those who do landscape photography this is not a big problem since most of the time the camera will be on a tripod.
But if you are shooting freehand you must take into account this loss of brightness, to avoid blur or micro-blur.
In general, good CPL camera filters reduce the incoming light by approximately one stop. But turning the filter can mean that the exposure compensation reaches 2 stops.
Unlike other lens filters, like neutral density filters, the CPL filter has a certain variability in the absorption of light.
For example, ND filters and GND filters have a set amount of filtered light based on how they’re built.
Usually photographic filter manufacturers indicate the ‘filter factor’ or compensation factor with an arithmetic scale: 1x, 2x, 3x, 4x etc.
For CPL usually there is a 2x or 4x filter factor. This means losing 1 stop of light or 2 stops of light (a half and a quarter of the incoming light, respectively).
Consequently, even if you do landscape photography on a tripod, you should keep this characteristic of the CPL filter in mind and adjust the exposure.
Remember that when you use multiple filters, the cutting multiplies.
When Using a Very Wide Angle Lens
Using a really wide angle lens can lead to halos in the sky. This ‘defect’ is not a true halo. It’s the correct polarisation of light in the central and upper part of the frame with a progressive loss of polarisation towards the edges.
This problem appears more often when using very wide angle lenses. Even 18-20mm on a full frame camera can lead to this.
But you can use this effect for creative photography, or to improve your landscape images in general.
I take most of my photos in the marine environment so I use a CPL filter to completely remove reflections from the water. I don’t care very much about the other components of the picture.
The CPL filter is an outdoor photography enthusiast’s best friend. The one indispensable too.
It can restore life to your photos and remove those small defects that the propagation of light brings into our cameras.
I never go on a shoot without it.
Add it to your Christmas wishlist this year, and you’ll be forever grateful.