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Polarizer

 

A polarizer truly is magic in a piece of glass and I never leave home without it. It is an expensive filter, but worth it all the money! It has several applications and, more importantly, it is impossible to simulate its effect afterwards on the computer. Some of the applications are discussed below.

 

Polarizer - clouds and the sky

 

The effect the filter is probably most known for is that you can manipulate the sky as a result of scattering (for more information on scattering and polarization, see under "the theory behind polarization" below). In the examples below, the blue sky (the scattered light which is polarized) is darkened because it is blocked by the filter, while the light which is reflected from the clouds is effectively unpolarized and thereby increases in intensity on the photo.

 

Polarizer - the sky at night

 

At nights with a bright moon, the sky scatters the moonlight in the same way as it does with sunlight during the day, which is why night shots turn up blue (unless, of course, it's a moonless night). But this light is also polarized, which means that a polarizer can make a big difference for night photography as well.

On the right is a test that I did, and it clearly shows the difference in the sky. The scattered moonlight is partially removed and the stars and the aurora become more apparent.

 

This is not a very useful application of the polarizer though.... Night photos are dark, and you need every bit of light for your photo, so it's not the best thing to put a filter on your camera that will take away 1-2 stops of light.

As a comparison, the pictures on the the right were both taken at f/5 and an exposure time of 20 seconds. However, the one without a polarizer was taken at ISO 1600, whereas I had to go up to ISO 6400 for the picture with a polarizer to get the land at approximately the same brightness.

 

Also, at night it is too dark to see the result of the polarizer directly through the viewfinder, so you'll have to take several images to find the optimal rotation of the polarizer.

 

 

 

 

 

 

 

 

Polarizer - reflections

 

Another use (and for me personally probably the most important) is its ability to block reflected light. Especially forest scenes really benefit from a polarizer in this regard. Because reflections are filtered out, objects like plants become more saturated. Below on the left is an example of this effect, where the photo where a polarizer was used has more saturation. It is also clear that it is important to have the polarizer at the right position, because it can also worsen a photo by actually enhancing the reflections.

Below on the right is another example of how a polarizer can drastically change the photo, just by removing reflections.

 

Polarizer - rainbows and mistbows

 

A polarizer can also be used to intensify a rainbow. This is a bit tricky though, a rainbow is polarized so the polarizer can also make the rainbow almost disappear when used at the wrong angle. You'll want the angle where the polarized light from the rainbow is transmitted without problems, whereas part of the unpolarized light (from everything else) will be blocked. This will intensify the rainbow as in the example below on the left, making the colors more vivid. Turning the polarizer 90 degrees will almost block the rainbow.

The same principle goes for a mistbow. Even though a mistbow doesn't have colors due to the smaller size of the droplets of a mistbow, a polarizer can still be used to intensify it. And turning the polarizer 90 degrees will also make the mistbow disappear, as in the example below on the right.

 

Polarizer - TFT screens

 

And finally, a bonus: this filter is also perfect for determining how clean your TFT screen is! On the left is a picture taken of my screen (I opened a blank text document so that my screen would be white) with a polarizer on my camera. Extremely boring of course, but watch what happens when you turn the polarizer 90 degrees. Since the light emitted from the screen is polarized, it is almost completely blocked if you turn the polarizer correctly. However, the dust on the screen scatters the light and changes its polarization so it ends up on your photo as white dots! Obviously, I needed to clean my screen.... (both pictures were taken at ISO 800, f/4 and 1/4 sec)

 

 

Since the light from a TFT screen is polarized, it can also be used as a source of polarized light. Taking a photo with a polarizer when holding a piece of plastic in front of your screen (displaying a blank white document) will show the mechanical stress in the plastic as different colors, due to birefringence. Birefringence is the effect that light of different directions of polarization behaves differently when passing through a material (the plastic in this case). In the example on the right, I used the cover of my LP player to take this picture.

Many more examples of birefringence can be found here.

 

Polarizer - the theory behind polarization

 

The principle is as follows, light usually is polarized in every direction and what this filter does is blocking part of these directions, as is illustrated in the scheme on the right. A polarizer can be rotated and this allows us to selectively block light of a certain polarization. The blue and red waves in the scheme are just two possible directions of polarization that I took as an example. Furthermore, direction of polarization has nothing to do with color, I arbitrarily took red and blue for clarification.

 

An important issue with light is that, when it is reflected, it does not do so in equal amounts for every direction of polarization. An example is given in the picture below, where the unpolarized incoming light hits a surface. The light which has a polarization parallel to the surface favors reflection, while the other directions favor refraction.

 

The effectiveness of this separation of polarizations is dependent on the angle of the incoming light. Only at a certain angle (called "Brewster's angle") is this process at its most effective, and will it render the reflected light as pure linear polarized light. The Brewster's angle is dependent on the medium on which the light is reflected (glas, water, etc.). Of course, all directions of polarization which are not reflected are refracted, so the refracted light is also slightly polarized (not shown in the schemes).

 

Below is an example of this principle, the left part is an open window and the right part is the reflection seen in the glass of the window. Turning the polarizer almost completely removes all reflection in the window.

 

It is important to remember that this does not apply for light which for example is reflected from clouds. In this case the light is not polarized and the reason is that a cloud is not a surface in itself, but a collection of an enormous amount of surfaces (the droplets) aimed at every direction. Therefore, the light from a cloud effectively is unpolarized, which is why a cloud can be intensified on a photo by using a polarizer.

 

Light also has the property to become polarized when it is scattered by small particles (dust, molecules, etc) and this so called Raleigh effect is the strongest at an angle perpendicular to the direction of the light. I've tried to clarify it with the scheme below, where the unpolarized light is again represented as the blue and red waves and the dust particle as the black circle. Light that comes from the left and continues in the same direction is unaltered in its polarization, it only loses some of its intensity due to the scattering. However, light which is scattered perpendicular to the original direction will be polarized (polarized in the direction of the red wave for horizontal scattering and in the direction of the blue wave for vertical scattering), whereas light scattered at non-perpencidular angles will be polarized only to a certain extent.

The best example of light scattering is the blue sky above us, this is sunlight scattered by all the particles in the atmosphere. That's also why the sky, when viewed from outside the atmosphere (e.g. from a space station), is no longer blue but pitch black, there are simply no particles to scatter the light. The reason that the sky is not white but blue is because blue light is scattered more effectively than red light because of the smaller wavelength of the blue light, which is also the reason why a sunset is red. Because the sun is low on the horizon during a sunset, the light has to travel a long way through our atmosphere (as opposed to during the day) and is scattered a lot. In fact, it is scattered so effectively, that blue light doesn't even reach your eyes anymore. Only the red light, which is less prone to scattering, can survive the scattering onslaught and thus we observe a red sunset.

 

Polarization by scattering is most effective perpendicular to the direction of the light, so a polarizer has the strongest effect when aimed at 90 degrees to the sun. The sky in the picture below on the left is a good example of how the effectiveness of a polarization filter differs depending on the angle to the sun. The part in the middle is polarized to a greater extent than the rest of the sky, and therefore becomes darker when photographed through a polarizer. This is an effect which is especially visible in pictures taken with wide angle lenses. Below on the right is another example, and there it can be seen that turning the polarizer removes the dark band.

 

Polarizer - circular vs linear polarizer

 

Two types of polarizers are available for use in photography: a linear and a circular polarizer. They both have the same effect on the photo, but most modern cameras work only well with a circular polarizer. To understand why, we can take a look at the schematic camera on the right (fortunately, my real camera looks a lot better than this one), this of course a simplified version and many parts have been left out for clarity. When light (the red beam) enters the camera through the lens, it will hit the mirror where it will be split in two. One part will reflect and go to the focusing screen (the line below the pentaprism) and pentaprism to form the image that we can see through the viewfinder. The other part will go through the main mirror and hits a second, smaller mirror which will direct the light to the autofocus detector (in the scheme, 1 is the pentaprism, 2 is the mirror, 3 is the sensor and 4 is the AF detector).

 

For correct exposure results it is important that the ratio between transmitted and reflected light is constant, but if polarized light enters the camera this will no longer be the case. As we have seen, light of different directions of polarization reflects differently. Where one polarization can reflect without problems, another one can be quenched almost completely. And the same thing happens when light hits the main mirror, some directions of polarization will reflect better than others which will more easily go through the mirror.

This is no problem with unpolarized light, but in the case of polarized light the polarization might be so that it favors reflecting on the mirror in stead of going through (example 1). Of course, the other scenario is also possible, where the polarization is such that it favors going through the mirror (example 2). Because of the different ratio of reflected/transmitted light in these situations, photos can end up under- or overexposed. And if that isn't enough, using a linear polarizer can also cause problems with the autofocus, resulting in unsharp pictures.

 

 

 

 

For this reason, a linear polarizer should be avoided for most modern SLR's and in stead, a circular polarizer should be used. This type of filter consists basically of two filters, a regular linear polarizer and a quarter wave plate. First, the light will be linear polarized by the linear polarizer, after which the quarter wave plate transforms it into circular polarized light, as is shown in the scheme. This circular polarized light no longer suffers from the problems with light metering and autofocus.

 

 

 

 

There is a simple test to determine which type of polarizer you have. In the case of a circular polarizer, when looking in the mirror through the filter a difference should be seen when you turn the polarizer around as is shown in the pictures below. If you see no difference then you have a linear polarizer and chances are that your camera won't operate well in combination with the polarizer. This test doesn't need to be done with a camera, it can also be tested by just looking through the filter in the mirror.

 

In the first situation, the light reflected from the camera first goes through the quarter wave plate, but since it is unpolarized, it does not change. Then, it goes through the polarizer and exits as linear polarized light before being reflected in the mirror. The polarized light reflected from the mirror has no problem going through the polarization filter again, since it has the same direction of polarization. Finally, it goes through the quarter wave plate again and reaches the eye or camera as circular polarized light.

 

In the second situation, the light will first become linearly polarized by the the polarization filter and then transformed into circular polarized light by the quarter wave plate. If we assume that its helix is right-handed before it hits the mirror, then it will be left-handed after reflection from the mirror and this makes a big difference. Since its helix is now the wrong one, the light will no longer be able to go through the quarter wave plate and you see no reflection in this case.

 

ND filter

 

A ND filter (where ND stands for neutral density, which is neutral grey) is nothing more than a (really) dark piece of glass, which can be used for several reasons:

• Normally, on a bright sunny day, it is very difficult to use large apertures, because it will result in overexposed pictures, even with the fastest shutter speed your camera has. So adding a ND filter will make it possible to use a large aperture on a bright day.

• Related to this is an effect which I find way more interesting, namely the use of long exposure times under conditions which normally would have short exposure times. This makes it possible to capture the movement of things which would have been frozen without the ND filter. The classic example of this is of course a waterfall, which becomes silky instead of frozen. But clouds are also great subjects to use with ND filters. Below are some examples of the use of ND filters, a 6 stop ND filter prolonges the shutter speed 64 times (2^6 = 64), a 10 stop ND filter with 1024 times (2^10 = 1024).

 

 

ND filter - viewfinder cover

 

When using ND filters, there is the possibility that light leaks in via the viewfinder, caused by the amount of light entering via the viewfinder being relatively large compared to the amount entering via the lens. The simple solution is to cover the viewfinder, and some cameras are actually equipped with a viewfinder cover.

 

Below on the left is a test I did on my camera, where I darkened the room, covered the lens with the lens cap and a black shirt, and exposed at ISO 800 for a long time, while shining directly on the viewfinder with a flashlight. Then, I repeated the test with the viewfinder covered. Clearly, light finds its way to the sensor under unprotected conditions!

On the right is a real life example, where the first picture is an exposure with a 10 stop ND filter attached. Sunlight from behind the camera gives a white haze in the middle of the picture, which can be circumvented by simply covering the viewfinder.

 

Gradual ND filter

 

A gradual ND filter can be used to compensate for a contrast difference in a scene which is too big to capture for the sensor. Both digital and analog photography is restricted to a limited number of stops which can be recorded in its dynamic range, whereas our eyes have a much bigger dynamic range. This is a reason why pictures can look totally different from what we observe, like washed out parts or black parts where we clearly saw details ourselves when we were there. This is a big challenge in photography and one of the things we can do about it is using a gradual ND filter.

 

There are many options for gradual ND filters, for example there are 1 stop, 2 stop, 3 stop filters and transitions can be hard or soft, depending on the conditions. If the scene has a relative abrupt transition from the light area to the dark area, you can use a hard filter, but in the case of a less well defined transition you can use a soft filter.

 

As the name suggests, the dark part of a 3 stop ND gradual filter is 3 stops darker than the light part, so only 12.5 % of the light will be transmitted in that part. Another way to see it is in exposure time. If you make the picture with a shutter speed of 1/10, then the dark part will effectively have had an exposure of 1/80 (three stops darker than 1/10). How strong the filter has to be differs of course for each situation, and this has to be done by determining the correct exposure for both parts and calculating the difference in stops between those exposures.

 

 

Gradual ND filter - reflections

 

A thing to keep in mind when using rectangular gradual ND filters (or any rectangular filter that leaves a gap between the lens and the filter for that matter) is that light coming from behind the filter can reflect back and show up on your photo as lighter corners. This is especially a risk when using wide angle lenses in combination with filters since a wide angle lens has such a wide field of view. If the reflections are only marginal, it might be possible to fix them in post-processing, but in other cases you'll be forced to crop the image to get rid of the corners.

 

The easiest way to prevent this from happening is simply blocking any light that might reach the filter from behind. There are special items for this, but I usually take a black piece of clothing (hat, glove, etc.) and put that in such a way so that it blocks the light. This is maybe not an ideal solution when it is windy, but it has worked for me so far.

 

On the right is an example of this type of reflections. Light coming from behind the camera reflected on the gradual ND filter and gave the lighter upper corners. Not a nice thing to discover when you get home and watch the photo on your computer.

 

 

 

 

 

 

 

 

 

Gradual ND filter - a poor man's gradual ND filter

 

Like any other filter, a good quality gradual ND filter is not cheap, especially considering that you'll want several of them for different situations. But there is a cheaper alternative, although its use is somewhat limited. By holding a piece of nonreflective black clothing (in my case, an old black sock with a piece of rectangular cardboard in it.... That's what I call state of the art!) in front of parts of the lens, you can simulate the effect of a gradual ND filter (it is more or less the same principle as burning). Credit where it's due: This is where I first found out about this technique. Below are two examples where I used this technique.

Very good results can be obtained with some practice, and in several cases, the results can actually be better than with a conventional gradual ND filter. This is because you have a lot of freedom in how to aim and move the clothing piece, and it is possible to adapt more precise to the conditions. For example, if the horizon is everything but a straight line, then this technique can be very suitable.

 

By changing the amount of time that you partly block your lens, you can change the intensity of the effect. The longer you'll hold it there, the darker the blocked part will become.

By changing the amount of movement during the blocking, you can control the sharpness of the transition. You'll get a sharp transition by not moving at all, and moving up and down a considerable amount will give you a soft transition.

 

The only disadvantage is that you'll need long exposures in order to obtain a smooth result, typically at least a second. Therefore, in some cases you'll be needing a ND filter to prolong the shutter speed. And as a result, moving objects (foliage, clouds, etc) will become blurred.

 

Don't be tempted to do this with anything else than a nonreflective black surface.... The picture on the right is the result of my first experiment with this technique and you can see a strong color cast in the upper half. That's because I used my hand to do this, giving a skin color tint to the upper half!

 

Combining filters

 

Filters can be combined for several reasons, and below are two examples where I used several filters together. One thing to keep in mind is that stacking filters will increase vignetting and possibly also the risk for flares.