Answers to the most common polarizer/UV questions

This document contains the answers to frequently asked questions about polarizing and UV/skylight filters. This document may be copied and distributed freely, as long as you keep in intact.

Polarizing Filters

Q1: What does a polarizing filter do?
A1: In order to answer this question, you'll first have to know something about light. Light can be regarded as a flow of particles (called photons), or as an electromagnetic wave. I'll try to stick to the wave theory. So, light can be regarded as a waveform, oscillating in an arbitrary direction perpendicular to its direction of motion. There will be waves oscillating up/down, oscillating left/right and all that's in between those two. A polaroid filter only allows to pass the component of the oscillation that is directed in the polarizing direction of the filter. So only the waves that are oscillating in the polarizing direction can pass the filter unattenuated. All other waves will attenuate according to the formula:
Amplitude after polarizing = Amplitude before polarizing * cos (theta),
where theta is the angle between the oscillating direction of the wave and the polarizing direction of the filter. The overall effect is blocking out half of the available light, and 'directing' the other half.
Q2: What is the difference between a linear and a circular polarizer?
A2: The main points are:
- If you use a standard linear polarizer with cameras that use auto-focus and/or auto-exposure, you may have problems. This filter may also cause trouble with manual cameras, if you're using TTL light measurement. A little dependent on the camera, your meter might go crazy. (It is not said you _will_ get problems, only that you might get them.)
- A circular polarizer can be used on all cameras, and will work the same as a linear polarizer with respect to darkening the sky, eliminating glare, et cetera -- circular polarizers are just more expensive.
A circular polarizer is just a linear polarizer followed by a quarter-wave plate set at 45 degrees to the axis of polarization.
A quarter-wave plate is made of a material in which light polarized in one particular direction travels more slowly than light polarized in the perpendicular direction. A quarter-wave plate is just thick enough that after passing through it, light polarized in one direction is delayed 90 degrees (or one-quarter wavelength) relative to light polarized in the other direction.
Since the quarter-wave plate is set at 45 degrees to the polarization, you can think of the incoming light as having two equal components in the principal directions of the quarter-wave plate. After passing through the plate, one component is delayed 90 degrees, and the resulting light is circularly polarized.
The idea is to use a linear polarizer up front to get rid of some linearly polarized light you don't want (glare off shiny surfaces, for example, will have a large linearly polarized component), and then it "stirs up" the result so you don't have linearly polarized light bouncing around in the camera.
A problem with linearly polarized light in your camera, for example, is that when you bounce it off a mirror at (near) Brewster's angle, it may be (nearly) completely eliminated. If the light meter measures the light after it bounces off a mirror, the amount of light arriving at the meter may be drastically different than the amount of light that will arrive at the film with no bounce, since the mirror has flipped out of the way.
Of course, a quarter-wave plate is only exactly a quarter wave for one frequency of light. That frequency is usually chosen to be a yellow in about the middle of the visible spectrum so that on the average, the light will be circularly polarized with various degrees of elliptical polarization mixed in. I suppose if you were photographing something that was primarily red, or primarily violet, your metering might be slightly off, even using a circular polarizer.
And of course, since there's another chunk of material in the way (the quarter-wave plate), there will be slighly more degradation of the image with a circular than with a linear polarizer.
Another nice way to think of circular polarization is to imagine a wave travelling down a rope where you hold one end and the other end is tied to a wall. If you shake your end back and forth along a line, the waves will all lie in a plane. You can shake your end in any direction perpendicular to the rope, and the only change will be in the direction of the polarization. Now start moving your end around in a circle, and circular waves will move down the rope. This corresponds to circular polarization. If you move your hand in an ellipse with various eccentricities, you'll get the equivalent of elliptical polarization (with various eccentricities).
If you're wondering whether your polarizer is circular or not, look through your polarizer at a mirror and look at how dark the polarizer is that the guy in the mirror is holding. Reverse the polarizer in your hand so the other side of the glass is pointing toward the mirror. With a circular polarizer, one direction will be significantly darker than the other. With a linear polarizer, both sould be the same. The reason is that linearly polarized light will still be linearly polarized in the same direction after bouncing off the mirror. Clockwise circularly polarized will be counter-clockwise after bouncing off a mirror, and will be cancelled when it comes back.
So if you hold a circular polarizer as if your eye is the camera (with the side that's normally screwed into the camera nearest your eye), it'll appear light in the mirror. If you flip it over it should appear almost black.
Some manufacturers (B+W and Heliopan, for example) sell a so-called Kaesemann polarizer which is even more expensive. A Kaesmann type has the foil stretched and held under constant tension in all directions. To do this it is necessary to totally edge seal the filter in glass rather than just bind the glasses and foil with an adhesive. This type of polarizer is available in linear, circular and in warmtone types. Its advantages are that the polarizing effect is slightly greater, the filter is "tropicalized" so it is immune to moisture, fungus, etc and it is very, very flat. So it will not adversely effect the sharpness of longer lenses. For this reason Heliopan only supplies Kaesmann type polarizers in sizes from 82mm up.
Q3: What can I use a polarizing filter for?
A3: The manufacturers will have us believe that you can block out any unwanted reflection in glass, water etc. You can *not* block out the reflections in metallic surfaces, since they do not polarize the light. But, although the manufacturers are right for the greater part, you will have to use the polarizer in the right way to get the above effect. This means, you will have to take your picture in a direction perpendicular to the sun (i.e. the line sun-reflecting surface has to be perpendicular to the line camera-surface), as is illustrated below:
```
    O  sun
     \
      \
       \      o  you
        \    /
         \  /
          \/
       ---------  reflecting surface
```
You will be able to block out the unwanted reflections this way, dependent on the direction of the filter. When you're standing perpendicular to the sun, the effect will be maximum, slowly decreasing as you move in line with the reflecting surface and the sun. Then the effect will become zero.
You can also use a polarizer to control the colour of the sky, ranging from light blue to dark blue/grey. Since the sky is scattered light and hence polarized, you can deepen the blue by removing light scattered by dust and molecules of, for example, water and hydrogen in the atmosphere(haze). In this way, you are able to let the clouds almost disappear or make them better visible. This also works best when you are standing on a line perpendicular to the line sun-earth.
Q4: What is the best way to work with a polarizer?
A4: It depends on what you are planning to do. When taking pictures of reflecting surfaces, it will give you the possibility to remove the reflections, thus creating a 'better' picture than without the filter. You can also use the polarizer to create more contrast in your pictures. The best way to find out what you can do with a polarizer is just try it. Use a roll of slide film (can't be corrected or ruined during printing) and take pictures of the things you normally take pictures of, but now use the following system: Take four or five pictures in a row of the same object, preferably with just a short period of time between them. The first picture should be taken without filter, just for reference. Then start with the filter in an arbitrary position, take a picture, rotate the filter a little (about 15 to 20 degrees ) and take the next picture, rotate the filter again etc. until you have four or five pictures. Then move on to another situation and repeat the above sequence. After developing the film, you will see quite remarkable differences between the various positions of the filter. Do try to start with the filter in the same starting position each time you start on a new series of four or five pictures. David Jacobson suggested you just look through the viewfinder of your camera, but that won't give you information on what influence the polarizer has on the autoexposure or autofocus of your camera (All cameras are equal, but some are more equal than others - after George Orwell). Also, there are compact cameras and TLR's with a possibility to append filters to them, and it is not much use looking through the viewfinder in those situations, since you're not looking through the filter. I admit, this is a situation that will not occur very often, but I wouldn't say it's impossible.
Q5: What effect does a polarizer have on taking pictures?
A5: As said before, a polarizer can influence the colours in your picture by darkening them, it can block out unwanted reflections and it can disturb your AF measuring beam or autoexposure (only linear polarizers). Also, because it will block about at least half the available light, it will slow your film down 1.5 to 2 stops, so if you are using a separate light meter, set your ISO dial 1.5 to 2 stops lower to correct for the loss of light. (You can also try measuring the amount of light through the filter with your light meter, but this is not a very accurate way of calibrating it). In this case, just try a few pictures, you'll soon find out what correction to use in your particular case. In case of doubt: A little overexposure is not as bad as underexposure, so if you want to be on the safe side, use the 2 stops correction. Warning: this is, if you're using normal film. Slides like to be underexposed a little if you're not sure on the amount of light. One other comment: the 1.5 to 2 stops is *not* valid for every polarizer. Most of them will indeed take 1.5 or 2 stops, but it may happen that your polarizer only takes 1 stop, or takes as much as 3 to 3.5 stops. It is totally dependent on the brand and kind of equipment you use.
Q6: How can I find out if I really have to buy a circular polarizer?
A6: It depends on the camera you're using if you need a circular polarizer or not. Most autofocus cameras have a semi-silvered mirror, and this can cause a significant difference in the amount of light reaching the photocell when using a linear or circular polarizer. Most of the manual- only cameras have their photocell in the prism, and they will not see the difference between linear and circular polarizers. So, in case of doubt, try a linear filter and -looking through the viewfinder- see if the reading of the light meter changes when rotating the polarizer. If it changes more than 1/2 stop, use a circular polarizer. This test has the most validity if you do it in daylight, looking at a grey wall.
Q7: Why do I keep getting reflections, even if I use a polarizer?
A7: Light which reflects off any surface is polarised to some extent. The degree of polarisation is related to the angle of incidence of light and the refractive indices of the two materials. At a certain angle known as "Brewsters angle", light is 100% polarised. At other angles of incidence the light is partly polarised.
Brewsters angle is given by
Brewsters angle = arctan( n'/n)
where
n' is the refractive index of the material giving off the reflection (eg glass, water).
n is the refractive index of the material through which the light is incident (eg air).
Refractive indices of common materials (relative to air, n=1)
```
    Material                 refractive index          Brewsters angle

    water                       n=1.333                     53

    glass                       n approx 1.5                56
                             (depends on the glass)
```
So enough theory, All the surfaces a photographer wants to control lie in the 50 degree range. Say you want to take a picture through a glass window. If you have no filter on you will see a reflection. If you put on a polarizer and take your picture looking straight through the window the reflection will still show up. But if you move around so you are looking through the window at an angle of 50 degrees, the reflected light will be 100% polarized. You then rotate the polarizing filter on your camera lens until the reflected image disappears. This is because the direction of polarization can vary with respect to the camera depending on the angle of incidence of the light.

UV/Skylight Filters

Q1: What does an UV or skylight filter do?
A1: Both of the filters filter out the UV light that can cause a blueish haze on your pictures, since normal film is not only sensitive to visible light, but also to UV. A skylight filter is also slightly coloured (pink or yellow), to give your pictures a 'warm' appearance (not so much blue).
Q2: What is the difference between a normal UV and a skylight filter?
A2: As mentioned above, the skylight filter is slightly coloured to give your pictures a 'warm' appearance.
Q3: In what situation do I use an UV filter?
A3: UV and skylight filters are useful when you are taking pictures in the mountains or at sea or any other place where there is a lot of UV light. It will filter out the blue haze that normally blurres the background of your picture. It is also very useful when taking pictures in the snow, since snow is a very good UV reflector.
Q4: Can I always leave the UV filter on the lens?
A4: About half of all the photographers keep an UV filter on their lens perma- nently, for it prevents your lens against dust, scratches and perhaps damage due to accidentally dropping the lens. A filter is much cheaper than a lens, so ruining your filter will not be as bad as ruining the front part of your lens. On the other hand, some photograpers (the other half) think it unneccesary to keep the filter on the lens, since everything between the original picture and your film, including filters, can cause blurrs or errors in the image, and that's one of the things we don't want to happen. It is just a matter of personal preference. I must admit I always have a filter on my lens, and I take it off only for cleaning.
Q5: What effect does an UV or skylight filter have on taking pictures?
A5: First of all, it will filter out most of the UV light. A skylight filter will also colour your pictures a little. Because the UV is filtered out, you may notice the sky in your pictures is not as deep blue as it used to be. This is because the UV component is now missing, resulting in another kind of blue on your pictures. Just try a few pictures with and without filter and see what you like best. An UV or Skylight filter doesn't have any effect on the amount of visible light falling through the lens, so you won't have to correct for it.