Polarizing filter

Learning goal

You will be able to explain why a polarizing filter darkens the sky and how it cuts reflections, and know at what angle and how far to turn it. You will describe how part of light is polarized light, vibrating in a particular direction; you will see that the sky darkens most at angles at right angles to the sun, and why it can darken unevenly with a wide angle; you will tell apart how reflections on the surface of water, glass and leaves are reduced with the filter, but why reflections from metal hardly change; you will take into account in exposure the light the filter will swallow; and while reducing shine on a face, you will take care, on every skin, not to erase completely the light that carries the face's shape.

The question

In the afternoon, you go back to the bridge you stood on the evening before. Below, on the river, a man is gathering his nets in a small boat. In the photo the sky is a pale blue close to white; the river's surface is a bright mirror of the sky, and the water around the boat is almost white. There is a small, bright spot on the boatman's sweaty forehead too.

How does the sky get bluer? A thin piece of glass that fits in front of the lens and can be turned can change this frame from the start: the sky darkens, the reflection on the water's surface disappears, and the stones under the water become visible. In this lesson we will talk about how a polarizing filter works, at what angle it works best, and where it needs to be used with care.

Context

In the black-and-white lesson, we saw that in the film era a red filter darkened a blue sky. A polarizing filter darkens the sky another way: not by cutting a color, but by cutting light vibrating in a particular direction. That is why it works in color photography too, and manages the sky and reflections without changing colors.

A polarizing filter is one of the few filters that editing programs can't easily imitate. It is possible to darken the sky in editing, but it isn't possible to erase a reflection on the water's surface and show the stones beneath it; the image of those stones isn't in the file at all. That is why a polarizing filter is a decision made at the moment of shooting.

Simple explanation

What is polarized light?

You can think of light as waves vibrating in every direction. Light coming from the sun vibrates in every direction. But part of this light starts to vibrate in a particular direction while it scatters in the air of the sky, or while it reflects off the surface of water, glass or leaves. Light vibrating in a single direction like this is called polarized light.

A polarizing filter is a thin layer that lets through only light vibrating in a particular direction. As you turn the filter, the direction it lets through turns too. When the direction of polarized light comes at right angles to the filter's direction, that light is largely cut: the sky's polarized brightness drops and the sky darkens, the reflection on the water's surface drops and what is beneath becomes visible. On cameras today, the kind called a circular polarizer is usually used; this kind is designed to work smoothly with the camera's autofocus and metering.

When does the sky darken most?

The sky's light isn't polarized to the same degree everywhere. The strongest polarization is in the sky at right angles to the sun, that is, in a direction making a right angle between you and the sun. If you look toward the horizon while the sun is beside you or overhead, the filter darkens the sky noticeably. When looking toward the sun or straight behind you, on the other hand, the effect is very weak.

That is why you need to be careful when using a polarizing filter with a wide-angle lens. While one end of the frame is at right angles to the sun, the opposite end is at another angle; part of the sky becomes dark navy, part light blue, and an uneven dark band appears in the frame. To notice this, it helps to look at how the sky spreads across the whole frame while turning the filter, and if needed to turn the filter back a little to reduce its effect.

How are reflections cut?

Light reflected from non-metal surfaces such as water, glass, shiny paint and wet leaves is most polarized when the surface is looked at from a particular slanted angle. When you look at water from above, at roughly a thirty or forty degree angle to the surface, the filter largely cuts the reflection: the stones beneath the river, the products behind a shop window, or the real green of wet leaves come out. When looking straight down at the surface, or almost parallel to it, the effect drops.

Light reflected from metal surfaces, on the other hand, is hardly polarized; the gleam on a car's chrome bumper or on the body of a teapot changes very little with the filter. And erasing a reflection isn't always what you want: the reflection of a mountain on a lake's surface, or the lights on a wet street, can be the subject of the photo. By turning the filter, you decide how much to cut the reflection; this reflection control doesn't have to be all or nothing.

How much light does the filter swallow, and how is it used on a face?

A polarizing filter also swallows part of the unpolarized light. Although it varies with the filter, this filter light loss is often between one and two stops. Because the camera meters through the filter, it adjusts the exposure by itself, but this loss lengthens the shutter speed or raises the ISO; in a dim place or with a moving subject, you need to take this loss into account.

Human skin reflects light too, and part of this reflection is polarized. A polarizing filter can reduce the shine on a sweaty forehead or a shiny nose. But this light on the skin also carries the shape of the face. On dark skin, the curve of the cheekbone and the forehead is often shown by these small glints; on light skin, too, a soft sheen adds life to the face. Turning the filter all the way and erasing all the shine can make the face look flat and dull on every skin. Finding a middle point that reduces the distracting shine and leaves the light that gives the face its shape often gives a more natural result.

Visual evidence

You go back to the bridge. The sun is on your right, and you are looking down at the river at a slanted angle. When you put on the polarizing filter and turn it slowly, the sky turns from pale blue to a deep blue and the white clouds stand apart from it. The bright reflection on the river's surface pulls back; around the boat, the round stones and moss under the water become visible.

The boatman, a dark-skinned old man, lifts his head and looks at you. When you turn the filter to its far end, the bright spot on his forehead disappears completely, but his face also looks flat and dull; the curve of his cheekbone can't be made out. You turn the filter back a little: the distracting spot on his forehead shrinks, a thin line of light stays on his cheekbone, and his face gains volume again. The exposure has lengthened by about a stop and a half because of the light the filter swallowed. The same bridge, the same river; the difference was made by how far the filter was turned.

Common mistakes

Summary

A polarizing filter darkens the sky and reduces reflections on non-metal surfaces by cutting polarized light vibrating in a particular direction; the effect is strongest in the sky at angles at right angles to the sun, and on water and glass at a slanted viewing angle. With a wide angle the sky can darken unevenly, metal reflections hardly change, and the filter often swallows between one and two stops of light. It can also be used to reduce shine on a face, but to avoid erasing completely the light that carries the face's shape on every skin, the filter needs to be stopped at a middle point.

Mini assignment

In this assignment you will try a polarizing filter at different angles and on different surfaces. If you don't have a filter, you can borrow one from a friend.

1. With the sun beside you, photograph the sky with the filter, turning it through a full turn, and compare the two positions where the sky darkens and fades.
2. Photograph the same sky while looking toward the sun too, and see the effect drop.
3. Photograph a water surface or a shop window from a slanted angle, with and without the filter; look at how much the reflection drops.
4. Photograph someone's face under bright light twice, turning the filter all the way and stopping it at a middle point; compare the shape of the face.

Check yourself

• Did you see the sky darken more at right angles to the sun?
• When you looked from a slanted angle, did the things behind the water or glass become visible?
• Which of the two frames of the face keeps its volume better, regardless of skin tone?

Glossary

Mini quiz

1. In which direction does a polarizing filter darken the sky most?

• A) Right in front of the sun
• B) In the sky at right angles to the sun
• C) Right behind the sun
• D) Equally in every direction

2. Why does the gleam on a car's chrome bumper hardly drop with a polarizing filter?

• A) Because light reflected from metal surfaces is hardly polarized
• B) Because chrome is very dark
• C) Because the filter only works on the sky
• D) Because the filter is on backward

3. When using a polarizing filter with a wide-angle lens, what causes an uneven dark band in the sky?

• A) The filter being dirty
• B) The lens's distortion
• C) The white balance being wrong
• D) Different parts of the frame being at different angles to the sun, and polarization not being even across the sky

4. While reducing the shine on a dark-skinned boatman's forehead, what do you do to keep the face's volume?

• A) Turn the filter all the way and erase all the shine
• B) Stop the filter at a middle point that reduces the distracting shine and leaves the light on the cheekbone
• C) Take the filter off and lower the exposure
• D) Set the white balance to the shade setting

Answers

1. B — The sky's light is most polarized at angles at right angles to the sun; the filter gives its most noticeable effect there.
2. A — Reflections from non-metal surfaces are polarized and cut; reflections from metal are largely not polarized.
3. D — With a wide angle, the ends of the frame stand at different angles to the sun; the difference in polarization leaves a dark band in the sky.
4. B — The small glints on the skin carry the face's shape; erasing all of them flattens the face.

Summary card

• A polarizing filter cuts polarized light; it is adjusted by turning.
• The sky darkens most at right angles to the sun; with a wide angle there can be an uneven band.
• Reflections on water, glass and leaves are cut at a slanted angle; metal gleam hardly changes.
• It often swallows one to two stops of light.
• Reduce shine on a face, but on every skin leave the light that carries its shape.

Bridge to the next lesson

In the bridge frame, the polarizing filter swallowed about a stop and a half of light, and the shutter speed lengthened a little. But when you want to soften the river's flow like silk, or blur the people in a busy square with a one-second exposure, this loss isn't enough in daylight; the light needs to be reduced much more. Balancing a bright sky with a dark foreground is a separate problem too. Is a long exposure possible in daylight? The next lesson talks about ND and GND filters.