What is a CPL Filter? How do you use it? The short answer is it’s like putting a pair of sunglasses on your camera. Colors pop. The sky becomes a bluer blue. Reflections that might lead to blown-out highlights melt away with a circular polarizing lens on the front of your camera.
This makes the circular polarizing lens filter an essential item in your camera bag. Deploying the CPL filter to the maximum advantage of the camera sensor is a little complicated but worth learning.
In this article, we look into just what the CPL filter does and how to make the polarization effect work for you. We’ll also post some photographs that show the results of using this valuable tool. And we’ll offer some tips for buying a CPL filter.
How Does a CPL Filter Work?
Digital photography sent many conventional filters into disuse. This is because modern photography software such as Photoshop and Lightroom copy their effects, especially color filters. And in some cases, photographers achieve an improved effect with a greater degree of control.
However, the circular polarizing lens filter occupies a special place in photography. Its effects are not so easy to replicate. These include color saturation, specifically the blues in the sky and the greens in the foliage. And in particular, reflections and glare from water, glass, or plastic resist post-production efforts.
The images below demonstrate the effects of polarization. The image on the left is unpolarized, while the image on the right is polarized.
The CPL filter reduces some types of light in such a manner that enhances a photograph. It cuts atmospheric haze and enhances the general contrast of a photo.
For a more detailed explanation of the circular polarizing filter, follow this link to Science World.
Circular or Linear
A circular polarizer filter takes its name from the manner in which it polarizes the light, not from the fact that its physical shape is round. It either blocks or allows circularly polarized light according to how the filter is oriented.
A linear polarizer passes either vertically or horizontally polarized light, again depending on how the filter is oriented.
However, a linear polarizer can create metering errors and autofocus glitches with modern DSLR or mirrorless cameras. So we will limit our discussion to the circular polarizer.
What is a CPL Filter?
Essentially, a CPL filter is a linear polarizing filter with a second optical element.
As the diagram shows, these two optical elements work together. The outer piece of glass, the element that you rotate, is a linear polarizing filter.
Between that and the camera lens lies a second glass element called a quarter-wave plate. This part attaches to the lens with screw threads.
The photographer increases or decreases the polarizing effect by rotating the linear polarizer filter.
This takes a little practice. The outer ring rotates continuously, in either direction, with no clicks and no 0-10 scale. But some brands include an arrow or other marker at the point of maximum impact on incoming light and the resulting image.
As you view the image quality through the viewfinder or LCD screen, the contrast and saturation of the image change as you rotate the outer ring.
How to Use a CPL filter?
The Angle to the Sun and the CPL Filter
The effect of a circular polarizing filter is greatest when the camera is at a 90-degree angle to the sun. The polarizing effect disappears when the light source is behind the camera. When you shoot into the sun, the effect also disappears, and the polarizing filter may cause an annoying lens flare.
Between those extremes, the CPL filter’s effect on light varies with the filter’s angle to the sun.
The photo of a wooden roof illustrates the polarizing effect. At the left is the image without a CPL filter. In the center image, the filter is at its minimum setting. And the image at right shows off the result at the CPL filter’s maximum polarization setting.
Use a CPL Filter on Longer Lenses
CPL filters work well on lenses with longer focal lengths. For example, a 100 mm lens limits the field of view compared with a wide angle lens. As a result, the polarizing effect is even across the frame.
However, wide angle lenses, and especially ultra wide angle lenses, present a problem with the CPL filter. The polarization effect varies with the camera’s angle to the light source – the sun. And a wide angle lens covers an expansive field of view. Therefore, areas of lighter and darker shades of blue in the sky lead to an odd-looking sky in an otherwise very nice landscape shot.
Some photographers avoid the circular polarizer filter on lenses wider than 20 – 25 mm. However, I experienced some uneven sky color saturation with a 35 mm lens. Plenty of factors come into play when you use a CPL filter. Getting to know how this filter works with its relationship to the sun maximizes its effect.
This photograph, taken with an 18 mm lens, shows an uneven effect of a CPL filter on a wide-angle lens.
The sky transitions from a natural blue to a faded blue from left to right.
Lenses with longer focal lengths minimize this effect.
Shutter Speed and the CPL Filter
Utilizing a CPL filter results in less light hitting your camera’s sensor, and you may have to reduce the shutter speed. Anticipate losing up to three stops based on the quality of the filter. Some situations, especially low light settings that call for long exposures, warrant the use of a tripod.
A Lens Filter for Protection
A filter on the front element of your lens, whether it be a UV filter or CPL filter, offers protection from dust, dirt, and smudges. And the filter is easily removed for cleaning with tools like these.
However, lens protection offered by a filter is limited. An unfortunate drop that bangs up the filter will potentially damage the lens as well, even if the damage is internal and not clearly visible. Lens protection beyond dust and dirt is not the job of a filter.
Travel and Landscape Photography
Photographers find the CPL filter useful in a variety of genres. But it excels in travel and landscape photography. Rotate the front element to get the desired effect. Notice the color boost of trees, the contrast between blue sky and clouds, and the richer tones on dry rock.
Blue skies are a deeper blue. Foliage takes on a richer green. Unwanted reflections in water surfaces fade away, and objects under the water’s surface become visible. You see an increase in clarity in the final image. Haze clears up and glare disappears from nonmetallic surfaces when you use a CPL filter.
The photographer in the photo above is dealing with haze and water reflection, common challenges in landscape photography that can be addressed with a CPL filter.
A circular polarizer darkens a scene to some degree while minimizing glare and removing glass reflections. This opens possibilities of low-contrast images with warm color tones. It allows the photographer to mitigate the effects of the strong midday sun to reduce reflections.
However, in the photo above, the CPL filter unlocks a world of eye-popping colors, from the kaleidoscopic color palette of the buildings to the deep blue sky to the rich tones of the water in the foreground.
City scenes encompass a wide range of reflective surfaces, including glass, chrome, and lights. Other surfaces like brick, concrete, skin, and fabric also benefit from the polarization effect.
Some of this can be attempted in post-processing. But the circular polarizer allows the photographer to preview the polarization effect in the camera with just the rotation of an index finger.
Reduce Glare When Shooting Through Glass
If you’re shooting through a window, you face the challenge of dealing with all the reflections. The circular polarizer filter involves extra effort and slows down the process, but it will reduce glare and remove reflections in those situations.
The photo of an airplane wing, taken from a passenger seat with a CPL filter attached to the camera, produces a clear view with no reflections from the window.
CPL filters excel at removing reflections, saturating colors, and managing extremes of shadow and light. For these reasons, this simple glass attachment becomes an essential part of a photographer’s gear.
Filter Fun With Rainbows and Waterfalls
For landscape photographers shooting rainbows, a CPL filter becomes essential. Rainbows result from the sun’s reflections off moist air. A circular polarizing filter rotated to boost this reflected light deepens the rainbow’s colors with exquisite results. Colors are saturated, and surrounding clouds are in high contrast. Rotated to the opposite extreme, the circular polarizer makes the rainbow fade or, in some cases, disappear. If that’s the image you’re looking for.
The photos of a rainbow and a waterfall show off the effect and versatility of the CPL filter.
Circular polarizers can do the job of neutral density filters in some settings. As mentioned earlier, the CPL filter results in about a 2-stop loss of light. Thus, it accomplishes part of what the ND filter does when it comes to long exposures. It allows a slow shutter speed to capture motion blur. It reduces glare from reflected surfaces and produces rich color tones. And you get this without the color cast common to ND filters.
Choose the Right CPL Filter for Your Purpose
Unless you already possess a filter holder system that includes a polarizer, I suggest a circular polarizing filter.
CPL filters come in almost any size. Major photography suppliers such as Amazon, Adorama, and B&H offer more than 20 filter sizes. Also, look for discounts and other incentives when you buy direct from manufacturers.
Determine the size of your camera lens filter thread. If you have multiple lenses with different filter thread sizes, buy a filter for the largest size. Then, purchase step-up ring adapters to attach the larger size filter to the smaller lenses.
What is the build quality? Do these filters affect the color temperature? Will it impact the camera settings?
Answering these questions comes down to testing multiple filters yourself, wading through product reviews, or relying on a brand name you trust.
Consider the Price
It appears that the correlation between the quality of photographic optics and their price is disproportionate. That is a small increase in quality results in a substantial bump in price.
I recently read a CPL filter review. Two filters, one at about $90 and another at $14 were compared side by side. The $90 filter emerged as slightly better but at more than six times the price.
Final Thoughts on Using CPL Filters
The polarizing filter avoided extinction in the digital age, at least so far, through its unique set of attributions. Its results on an image resist duplication in post processing software, although the Dehaze slider in Lightroom is working on that.
It brings versatility to many challenging settings in photography. Color saturation, subtle reflections without the blown highlights, striking contrast, and in-camera control make the CPL filter an essential component in photography. And well worth the price.