CPL Filter: Why, When, and How to Use a Circular Polarizer

A CPL filter (or circular polarizer filter) is an effective and popular filter which can decrease or eliminate reflections and glare from your photos. In this article, we will introduce this filter type and analyze its use and effects.

Table of Contents

What is a CPL Filter?

Have you ever seen beautiful summer photos of boats floating on crystal-clear waters and wondered why your photos captured at the sea do not look as clear or pretty? How can you capture photos that show the depth of the sea?

Well, the answer is very easy: this effect can be achieved by using a CPL filter. A CPL filter is a glass filter that reduces or eliminates the glare from reflective surfaces in photos.

A NiSi Ultra Slim Pro MC CPL filter.

It is very important to note at this point that even with today’s advances in technology and AI software, it still remains the one filter effect that cannot be reproduced digitally.

Basically either you will capture the scene with the CPL filter removing any unwanted reflections at that time of the capture… or not. Reflections captured in a photo cannot be magically eliminated from the scene with a photo editing app.

On some occasions, it might be possible to remove some unwanted reflections, for example from glasses on a studio portrait, but that will definitely require a lot of time in post-processing.

Let’s see some before and after examples:

Before
After. Notice the increased clarity in the water due to the reduced reflections.
Before
After. The reduced amount of haze makes the mountains and cliffs appear clearer.

The Design of CPL Filters

A CPL filter consists of two slim metallic frames that are very close to each other. One piece of ND glass is held in each frame.

Note: A CPL filter can also act as an ND filter with a blocking strength of 1-1. 5 stops.

The first optical element is placed on the metallic frame, which is attached to the lens filter thread and is static, while the second optical element is placed on the moving outer frame, which gives us the ability to rotate it based on the position of the camera and lens.

Why should the front element turn?

The rotating glass on the front has a linear polarizer that filters the unpolarized incoming light, transforming it into linear. Basically, as light literally comes from all directions, this glass contains many tiny lines that will block specific waves of light, letting only the waves of the X-axis pass.

Light passing through a linear polarizer. The unpolarized beam enters the filter and a beam with a single linear polarization comes out. Illustration by Bob Mellish and licensed under CC BY-SA 3.0.

The static element on the back is a quarter-wave plate glass that transforms the passed linear polarized light into circularly polarized light.

The addition of a quarter wave plate creates circularly polarized light that does not cause issues with camera equipment. Illustration by Dave3457.

This must happen in order to avoid any possible issues with the autofocus systems of our cameras and also any possible misreading of the camera light meter. Linearly polarized light can cause issues with both systems.

During the analog era of cameras, when autofocus systems had not been introduced yet, most polarizer filters were linear only.

How a CPL Filter Affects Photos

In addition to cutting out any possible reflections and glare directly on the scene, a CPL filter also:

  • Increases the overall contrast of the image.
  • Enhances overall colors (saturation) and especially those beautiful blue skies and in foliage.
  • Reduces atmospheric haze.

Here are some sample photos captured using a circular polarizer filter.

How to Achieve the Optimal Polarization Effect

To achieve the optimal polarization effect, the light source (the Sun, for example) must be located at an angle of 90deg degrees from the point where the camera is looking.

An easy and very practical way to calculate out in the field if you can achieve the maximum polarization is to use the rule of thumb.

Your index finger should look the same way as the camera and your thumb must target the sun. :

So practically during the summer season when we have the chance to visit some beautiful seascapes, midday is the best time to capture crystal clear waters of the scenery, as in the photo below:

Crystal clear waters at Antiparos Island, Greece.

Some filters have also a marked point on their frame which indicates the proper position to have an optimal polarization effect.

The Side Effects of CPL Filters

A CPL filter is a must-have tool in the camera bag of a photographer/videographer and in most cases is extremely useful, but there is an unwanted optical effect that can be produced when a CPL filter is used with an ultra-wide lens (with a focal length below 24mm in full-frame format).

For example let’s see the following picture:

In this case, a lens with a focal length of 20mm was used and as we can see, the filter couldn’t maintain the polarization effect in the whole sky, having produced uneven darker parts in the sky, and more specifically in the side of the image which was far from the sun.

Note that this has nothing to do with the CPL manufacturer or build quality — it’s an optical phenomenon that inevitably will happen.

Also, as with many circular filters, some vignetting may be noticeable if your filter’s metallic frame is not thin enough.

When is a CPL Filter Useful?

The most common use of a CPL filter is in landscape and travel photography, as it can enhance the image drastically on the fly. You can simply rotate the front to reveal the polarization effect. This will give you enhanced colors as well as a higher contrast image.

A real-time look at the effect of rotating a CPL filter on a scene.

Especially during autumn and spring is ideal as it can create very beautiful tones on the produced image.

A Cycladic orthodox church in Paros island, Greece.

Another very common use of a CPL filter is when we need to photograph through glass.

For example, to photograph a shop or house through the window. You can also view the city from an observation deck on a skyscraper’s top floor.

The view from observation deck on floor 69 of the Shard, London.

As its main effect is to cut reflections, it is very useful in product photography, especially when we photograph jewels and watches.

When Not to Use a CPL Filter

When should you not use a CPL filter for your photos? Mostly when your goal is to capture reflections that would be reduced or eliminated by the filter.

You might also consider not using a CPL filter when you are shooting in low light conditions. The filter’s light-blocking properties could cause it to be difficult to expose your scene properly.

As stated earlier, you may also choose not to use a CPL filter when using ultra-wide lenses to avoid creating uneven darkened areas in the sky.

Conclusion

As we mentioned in the beginning, the CPL filter effect still remains something that even the most advanced software cannot reproduce accurately (at least at the time this article was written), and we consider this filter one of the most important for every photographer.

It is an easy, inexpensive way to improve the quality of your photos. We recommend it in every camera bag regardless of the type or genre.

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