This file contains the answers to frequently asked questions about polarizing
and UV/skylight filters. I saw so many questions about these filters on news,
I thought it wouldn't hurt to make it a FAQ.
If anyone has comments on this faq or useful supplements, please E-mail them to


All the information useful to other news-readers will be added to this FAQ,
mentioning the name of the one who sent it to me. You can also E-mail to the
above addresses if you still have unanswered questions about filters. Should 
there still be questions that are frequently asked, I'll add them to this FAQ.

This file may be copied and distributed freely except for direct commercial
use, as long as you keep in intact.
So let me get the credit for the time I spent on it and do not just copy parts
of it, for chances are that I will get questions that are already answered in 
this FAQ.

This FAQ will be posted regularly, at least once a month, with an expiring time
of approx. 2 weeks.


> update september 28, 1994

- The answer to Q2 of the polarizing filters has been replaced by the FAQ on
  circular polarizers written by TOM DAVIS (

- DAVID JACOBSON ( suggested I should go for the
  wave theory on light instead of the particles. I tried. He also had some
  useful comments I have added to this FAQ, and he pointed out an error in A5
  on the polarizers. I was wrong on the amount of light you loose, but it has
  been corrected now.

- RICHARD KARASH ( also had some comments on the circular
  polarizer, but they were already in the circular polarizer FAQ by Tom Davis.

  ( also commented on the polarizers, mostly about the
  circular vs. linear question, and dr. Larkins commented on my advice of
  overexposing film when in doubt of the light. This is corrected. The other
  comments on the circular polarizer were already corrected in the circular
  polarizer faq by Tom Davis.

> update september 29, 1994

- LUCA DE ALFARO ( sent in a test about determining if you
  really need a circular polarizer. Jeff Spirer sent a comment on this, so I
  decided to unite both of them in Q6 and A6 on polarizers.

> update october 3, 1994

- JOE CALI ( sent in an explanation of the so called
  "Brewsters Angle". This is added to the FAQ in Q7 and A7.

> update october 4, 1994

- Rev. Dr. PHIL HERRING ( pointed out that even if you 
  use a manual camera, a linear polarizer might disturb your light meter. This
  is added to A2 on polarizers

> update november 28, 1994

- A.T. Young ( suggested that instead of shooting a roll
  of film just to see the effects of polarizers, we should take a look at the
  book "Polarized Light in Nature", by G.P. Ko"nnen (sorry, the computer won't
  allow alt-148), which gives you a lot of information and shows you a lot
  without having to shoot even one picture. Of course, for those who cannot
  find the book (me, for example), keep trying yourself, or try to find another
  book on this subject.
  Also, he gave the comment that at a refractive index of 1 there's no
  reflection, so 'air' should be removed from the list in Q7/A7. I did, but 
  keep in mind all the refractive indices are relative to air (n=1).
  Last, he pointed out that the sky is not reflected light but scattered light.
  This point is corrected in Q3/A3 on polarizers.

> update april 19, 1995

- I saw a thread about the advantages/disadvantages of using multiple filters at
  the same time, so I added Q6/A6 to the UV filters part. I tried to use all the
  comments I saw in the newsgroups, leaving the ones that were really wrong. You
  can see for yourself how well I managed doing this. Because the answer is a 
  compilation of comments from different people, I can't give credits for it to
  one person.

> update july 14, 1995

- I noticed a question about the use of polarizers and non-uniformly coloured
  skies when a polarizer was used. I also read some comments on this that it
  might have been caused by vignetting, when you are using a wide-angle lens.
  I combined this question and comments in Q8/A8 on polarizers. Again, because
  the answers came from a number of people, no single person can get the credit
  for it.

* I want to thank all people who commented for their contribution to this FAQ. *

    - 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

    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:

    (1) 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.)

    (2) 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

    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

    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)

	    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

Q8: When I use a polarizer and a wide-angle lens, the sky seems darker at the 
    edges of the picture. What is this effect and what can I do to prevent it?

A8: There are two explanations possible. If you're using a very wide angle lens
    (28 mm or less), there's the possibility of vignetting because the lens can
    'see' the edge of the filter. When your pictures suffer from vignetting, all
    4 corners of the picture will be darker than the rest of the picture. There
    is only one good solution to this: work without the filter if you do not
    really need it. If you really need the filter, make sure you use a very 
    thin filter, or a filter so much bigger than the lens it is mounted on, that
    the edges will not be visible.

    The other possible explanation, when you're sure your pictures are not
    suffering from vignetting (when only the 'sky-corners' are darker) is, that
    when you use a wide-angle lens, the polarizing effect of the filter when
    viewed from the lens is not the same everywhere, because the polarization of
    the sky itself is not uniform. So, the more sky on your picture, the better
    your chances are for getting a non-uniform coloured sky. The only cure for 
    this is using a longer lens instead of wide-angle lenses, for there's almost
    nothing you can do against this 'polarization-mismatch'.

    - 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.

Q6: Do I have to remove my UV filter when I want to use another filter, such as a

A6: It is not necessary to remove your UV filter when you're going to use any
    other filter you may have, but it won't do any harm. Some filters, like
    polarizers, do cause image degradation because of their construction. This
    degradation is usually more than the degradation alreadycaused by the UV
    filter, so in those cases you do not really have to remove the UV filter.
    On the other hand, you run the risk of light being 'trapped' between the two
    filters, thus causing lighter spots on your picture.
    When you're using filters of very high optical quality, it's always a good idea
    to remove all other filters. In general, the less filters on your lens, the
    better the quality of the image.
    Another thing to remember: multiple filters on your lens may cause vignetting,
    especially when you're using a wide-angle lens. So decide for yourself whether
    the saving of time by not removing your UV filter before mounting another is
    worth the trouble you may experience from all kinds of nasty effects on your

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