The filter polarizing filter is one fundamental that every photographer should carry in your bag. In fact it is possibly the single filter whose effect can not be simulated by software . The polarizers are used to increase contrast and color saturation to intensify blue skies and, more importantly, to reduce or eliminate reflections from glass, water and other surfaces, but how they do it? What's the difference between a linear polarizer circular polarizer?

To understand how a polarizing filter must understand the nature of light. A beam of light traveling in a straight line, it does infinite planes and swings around the axis that marks the direction of travel. Each of them is a plane of polarization and polarizing mission is to stay only one or a narrow range.

For easy understanding, the polarizer is often likened to a grid, which can only pass the waves are aligned. Thus, as we turn the filter select a plane of polarization or the other.

Operation of the linear polarizing filter.

When we met in front of a reflection surface (except metal), these reflections are composed of partially polarized light. If we rotate the polarizing filter properly, we can stay with a single plane of polarization and, in doing so, eliminate glare.

As you may have been deducted, if we eliminate light means that the polarizing filter affects exposure and also varies the amount of light that remains as its angle of rotation, accepting that remains between one and two diaphragms according to their position.

How to remove reflections from water or a glass polarizer

If you try to take a picture of a window or a glass-covered box and there is light at your back, you will soon see your reflection in it. The solution is the polarizing filter, but you can not put you to work in front of the glass, but you must put yourself at an angle of 45 degrees. After looking through the viewfinder and rotate the polarizing filter to reduce the reflection as possible.

In the case of the reflections in the water would do the same, except that in this kind of pictures and it is generally forming an angle with the surface of water.

Thanks to the polarizer, the photo on the right has less reflections in the water.

Improving the sky with the polarizer

If the sky of your landscape is often clear out, the polarizing filter can help, but keep in mind that not always behave the same. If you point the lens directly into the sun, or just let the sun at your back, the polarizer will not help.

In contrast, when the sun is right next to yours, and the target points perpendicular to its path, the polarizing effect can be dramatic enough to get almost black skies at two in the afternoon, although this can always be reduced by turning some leave the filter in an intermediate position between its minimum and maximum effect.

The photo at right, with the polarizer, shows a deeper blue in the sky.

If you use an objective very angular (usually landscapes) should be careful with the polarizer, as in the same frame can be collecting the sun's path and its perpendicular, making heavy use of the filter would darken only part the sky, which usually gives a result very little aesthetic.

Roll your mouse over the photo to see the effect of polarizing

Increasing the contrast and saturation with the polarizer

Easier to use polarizer is to increase the contrast and color saturation. Just turn it while looking through the viewfinder, until the image looks the one you like. This is especially useful for increasing the volume of the clouds, or the color of vegetation.

The reason for the increased saturation is that, as with water or glass, we are removing some of the reflected light.

Sometimes you have to find a compromise solution, because in one scene, for example, sky and water reflections, it may happen that the position of the polarizer that is good for the sky is not for water, and vice versa.

"Linear or circular polarizing polarizer?

So far all this is applicable to any polarizing filter, but when you buy you should be aware that there are two types: linear polarizer and circular polarizer, and this has nothing to do with its shape (square or round), or with the fact that spin (all do), but the way they work.

The sensors of modern cameras, so the meter, such as autofocus, are affected by the polarization of light, so if they get light oscillating in a single plane, whichever is the plane, may provide erroneous measurements, which would result in poorly exposed photos and / or focus.

To avoid resort to change the type of polarization, from linear to circular. This will simulate all the planes of polarization by rotating the one who has missed the filter. This is achieved by placing a polarizer after the delay filter λ / 4 (so named because it causes a lag of π / 2, which is the fourth part of a full wavelength: λ = 2π), so we filter the planes that do not interest us, and we stayed at the offset, so we rotate so that the sensors are not affected. The binding of a polarizing filter and a filter λ / 4 is a circular polarizing filter, and is to be used in any modern camera.

Operation of the circular polarizer.