Do we really need a CPL filter？Since the CPL filter is a circular polarize

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Do we really need a CPL filter？The CPL filter is a circular polarizer, and its effect is the only one that can not be easily finished during post-process in the digital photography era.  The opposite of the circular polarizer is the line polarizer, but it seems that there is basically all circular polarizer on the market. Therefore, let me quote the results of the mechanism of circular polarizer from the internet.“A polarizing filter or polarising filter (see spelling differences) is often placed in front of the camera lens in photography to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections (and sky-light) tend to be at least partially linearly-polarized, a linear polarizer can be used to change the balance of the light in the photograph. The rotational orientation of the filter is adjusted for the preferred artistic effect. For modern cameras, a circular polarizer (product labeling abbreviation: CPL) is typically used; this comprises firstly a linear polarizer which performs the artistic function just described, followed by a quarter-wave plate which further transforms the now-linearly polarized light into circularly-polarised light before entering the camera. This additional step avoids problems with auto-focus and light-metering sensors within some cameras, which otherwise may not function reliably with a simple linear polarizer.”

“Use of a polarizing filter, in the correct direction, will filter out the polarized component of a skylight, darkening the sky; the landscape below it, and clouds will be less affected, giving a photograph with a darker and more dramatic sky, and emphasizing the clouds.[4] Perpendicularly incident light waves tend to reduce clarity and saturation of certain colors, which increases haziness. The polarizing lens effectively absorbs these light waves, rendering outdoor scenes crisper with deeper color tones in subject matter such as blue skies, bodies of water and foliage.”

Do not need to be considered too much about the above principles. We only need to know that the effect is changing with the rotation of the filter. So what is the outcome of the polarizer?

1, Filter the polarized light in the sky to make the sky purer and stronger comparison with the white clouds.

2, Eliminate the reflection on the surface of transparent objects such as water surface and glass.

3, Eliminate the reflected light from the surface of the object.

4, Work as a ND lens that reduces the amount of light in two level.

The pictures below except Scene 1 were taken after a new CPL was recently installed. Although some of them have the same color temperature due to the auto white balance.

Scene 1: Make the sky bluer

Although the above picture is a wrong demonstration, the middle part of this photo is obviously bluer. This is the one can show the effects of the polarizer clearly. However, this kind of demand is not recommended to use polarizers. It will make the sky darker, and the blue result is natural to do on the computer. Besides, uneven images are easy to appear when used on wide-angle lenses like the picture above.

Scene 2: Eliminate the reflection of transparent objects such as water and glass

In the filtered photos from the above, the reflection of the water is filtered by the polarizer. The stems and leaves under water are more transparent to see.

Scene 3: Eliminate the reflected light from the surface of the object

When the light is filtered, the reflected light on the lotus leaf is eliminated, the whitening condition is effectively suppressed, and the lotus leaf is greener. However, the CPL is not always useful. The best situation is when the light has 45 degrees with the lens, and the backlight or the 90-degree light at noon is basically ineffective. ( However, you still can use it as a ND lens.

But in fact, this effect needs to be controlled by the angle of the rotation of the filter; otherwise, the filtered photo as the second example will be a bit distorted. In the first example, the strongest polarizing effect was not used, and the result was pleasing to the eye. In this case, also pay attention to the camera's white balance setting, the auto-white balance may cost color shift, pay attention to the setting or correct in the later stage.

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