Playing with polarisers

ben brugman wrote:
> >> It has stopped raining so I will pop out for a few photos before
> >> dinner.
>
> Polarizers are often very effective imediatly after the rain.
> Often you get patches of blue sky. And everything is wet,
> great to try a polarizer in those circumstances.

So I have read but I didn't get anything good tonight. Oh well, keep
trying - practice makes perfect.

--
Seán O'Leathlóbhair
 >> Stay informed about: Playing with polarisers 

"Seán O'Leathlóbhair" <jwlawler.RemoveThis@yahoo.com> wrote in message
news:1180446046.895814.242270@q75g2000hsh.googlegroups.com...
I just bought a couple of circular polarising filters. Both are
Hoya. I wanted one to improve my landscape photos. The reason for
two is that I was dithering as to which lens to buy it for when the
guy in the shop offered me a good discount for the two. My camera is
a Canon 300D. The lenses being considered for polarisers were the:
18-55mm kit lens and the 28-135mmm IS lens. The 28-135 should be the
better lens but I am quite happy with the kit lens and I like the
extra width for landscapes. Anyway, problem solved, I now have both
sizes. One will also fit my 75-300mm but it would rather dark at the
300 end so that may not be very useful.

Question 1, the instructions on the box are minimal. There is an arrow
on the rotating ring. When this is at the top does it mean that
vertical polarisation is selected?

Since I had two filters, I took them out of their boxes and looked
through them. As I rotate them, they do change (so it seems that they
really are polarisers) but not quite as I expected. The change seems
to be more of colour than density. When lined up the filters appear
slightly orangey and when 90 degrees apart they appear grey but not as
dark as I hoped. If they were perfect, I would expect black. I guess
that is hoping too much but I did expect a darker grey.

Has this experiment produced unexpected results because these are
circular polarisers? Maybe, since the first will output circularly
polarised light, I should not expect rotating the second to have any
effect at all. This has just occurred to me, if I turn one around so
that they are face to face, should I get the effect that I expect?
Should I find a linear polariser or a pair of Polaroid sunglasses and
put it in front of one of my filters to test it?

Of course, another test is to put the filters on the camera and go and
take some pictures. The problem with that is that the weather is
lousy here and it is raining as you may expect for November rather
than late May.

--
Seán O'Leathlóbhair

Hi,
A circular polarizer has a quarter wave retarder behind the main polarizer
to circularly polarize the light coming off the regular polarizer. It's
purpose is to allow proper exposure on cameras that send the light through a
partially silvered mirror as many AF cameras do. The quarter wave retarder
causes the polarizer not to seem to look right when peering through a couple
of them. Place the FRONTs of the to polarizers facing each other and turn
the. You should see the polarizers work as they should when you turn one.

For anyone in the US who is near a Meijers store. For $13 each they sell
"Targus" branded CPL filters in 52 and 58mm sizes. My tests shows they work
just fine, but they don't include a case. Local camera store wants nearly
$40 for a "Promaster" brand (rebranded Hoya).
John
 >> Stay informed about: Playing with polarisers 

>> It has stopped raining so I will pop out for a few photos before
>> dinner.

Polarizers are often very effective imediatly after the rain.
Often you get patches of blue sky. And everything is wet,
great to try a polarizer in those circumstances.

ben

>>--
>>Seán O'Leathlóbhair
 >> Stay informed about: Playing with polarisers 

This message is not archived
 >> Stay informed about: Playing with polarisers 

The practical use of polarizing filters is empirical, depending on what you
see through your SLR rather than where little marks on the ring are. The big
initial surprise will be that the degree of polarization varies depending on
the angle of the camera vs the position of the sun. This will be a big
problem for really wide angle shots, where polariaing filters don't work so
well, because of an ugly variance of darkness across the sky. You can
estimate the area of the sky most prone to dramatic polariaing effects. It
is 90 deg off the position of the sun. To find that spot, point your index
finger at the sun, and your thumb will be pointing to the area of the sky
that will be most affected by polariaing filters. Of course it is a circular
line across the sky, partially obscured by the earth that gets in the way.

Dave
 >> Stay informed about: Playing with polarisers 

Seán O'Leathlóbhair wrote:
> JohnR66 wrote:
>
>>"Seán O'Leathlóbhair" <jwlawler RemoveThis @yahoo.com> wrote in message
>>news:1180446046.895814.242270@q75g2000hsh.googlegroups.com...
>>I just bought a couple of circular polarising filters. Both are
>>Hoya. I wanted one to improve my landscape photos. The reason for
>>two is that I was dithering as to which lens to buy it for when the
>>guy in the shop offered me a good discount for the two. My camera is
>>a Canon 300D. The lenses being considered for polarisers were the:
>>18-55mm kit lens and the 28-135mmm IS lens. The 28-135 should be the
>>better lens but I am quite happy with the kit lens and I like the
>>extra width for landscapes. Anyway, problem solved, I now have both
>>sizes. One will also fit my 75-300mm but it would rather dark at the
>>300 end so that may not be very useful.
>>
>>Question 1, the instructions on the box are minimal. There is an arrow
>>on the rotating ring. When this is at the top does it mean that
>>vertical polarisation is selected?
>>
>>Since I had two filters, I took them out of their boxes and looked
>>through them. As I rotate them, they do change (so it seems that they
>>really are polarisers) but not quite as I expected. The change seems
>>to be more of colour than density. When lined up the filters appear
>>slightly orangey and when 90 degrees apart they appear grey but not as
>>dark as I hoped. If they were perfect, I would expect black. I guess
>>that is hoping too much but I did expect a darker grey.
>>
>>Has this experiment produced unexpected results because these are
>>circular polarisers? Maybe, since the first will output circularly
>>polarised light, I should not expect rotating the second to have any
>>effect at all. This has just occurred to me, if I turn one around so
>>that they are face to face, should I get the effect that I expect?
>>Should I find a linear polariser or a pair of Polaroid sunglasses and
>>put it in front of one of my filters to test it?
>>
>>Of course, another test is to put the filters on the camera and go and
>>take some pictures. The problem with that is that the weather is
>>lousy here and it is raining as you may expect for November rather
>>than late May.
>>
>>--
>>Seán O'Leathlóbhair
>>
>>Hi,
>>A circular polarizer has a quarter wave retarder behind the main polarizer
>>to circularly polarize the light coming off the regular polarizer. It's
>>purpose is to allow proper exposure on cameras that send the light through a
>>partially silvered mirror as many AF cameras do. The quarter wave retarder
>>causes the polarizer not to seem to look right when peering through a couple
>>of them. Place the FRONTs of the to polarizers facing each other and turn
>>the. You should see the polarizers work as they should when you turn one.
>
>
> <snip>
>
> Yes, if I turn one polariser around, they behave as expected: when
> lined up, they look pale grey, the same as one by itself; when one is
> rotated 90 degrees, they go very dark, not completely black but a very
> deep purple.
>
> For a laugh, I tried them back to back, there is no noticeable
> difference at all as you turn one. Considering the circular
> polarising layer this makes sense.
>
> The effect in the original orientation is still a puzzle: a change in
> colour not density.
>
> Anyway, I am happy now that I have two functioning polarisers.
>
> It has stopped raining so I will pop out for a few photos before
> dinner.
>
> --
> Seán O'Leathlóbhair
>
Circular polarizers are not perfectly achromatic. That is, the
quarter-wave plate is only a quarter-wave plate at one wavelength,
unless they use a very expensive achromatic quarter-wave plate, which I
doubt. (I've had them made for my research and paid $1000s.) So the
linear-to-circular conversion is only "perfect" at one wavelength, and
as you depart from that wavelength, some residual linear polarization
will remain. Even the purely linear polarizers are not completely
achromatic (unless they are very expensive) and will not give complete
cancelation when crossed for all wavelegths; often blue light leaks
through, though it can be red. The result is that, if you cross
polarizers of various kinds in various ways, you can get some funny
color effects. Two successive circular polarizers can give these color
effects for these reasons. The first one will "leak" some
wavlength-dependent linear polarized light that will interact with the
second polarizer to various degrees, depending on its orientation.

I hope this helps.

Joe
 >> Stay informed about: Playing with polarisers 

On 30 May, 22:39, Joseph Miller <mil... RemoveThis @ucolick.org> wrote:
> Seán O'Leathlóbhair wrote:
> > JohnR66 wrote:
>
> >>"Seán O'Leathlóbhair" <jwlaw... RemoveThis @yahoo.com> wrote in message
> >>news:1180446046.895814.242270@q75g2000hsh.googlegroups.com...
> >>I just bought a couple of circular polarising filters. Both are
> >>Hoya. I wanted one to improve my landscape photos. The reason for
> >>two is that I was dithering as to which lens to buy it for when the
> >>guy in the shop offered me a good discount for the two. My camera is
> >>a Canon 300D. The lenses being considered for polarisers were the:
> >>18-55mm kit lens and the 28-135mmm IS lens. The 28-135 should be the
> >>better lens but I am quite happy with the kit lens and I like the
> >>extra width for landscapes. Anyway, problem solved, I now have both
> >>sizes. One will also fit my 75-300mm but it would rather dark at the
> >>300 end so that may not be very useful.
>
> >>Question 1, the instructions on the box are minimal. There is an arrow
> >>on the rotating ring. When this is at the top does it mean that
> >>vertical polarisation is selected?
>
> >>Since I had two filters, I took them out of their boxes and looked
> >>through them. As I rotate them, they do change (so it seems that they
> >>really are polarisers) but not quite as I expected. The change seems
> >>to be more of colour than density. When lined up the filters appear
> >>slightly orangey and when 90 degrees apart they appear grey but not as
> >>dark as I hoped. If they were perfect, I would expect black. I guess
> >>that is hoping too much but I did expect a darker grey.
>
> >>Has this experiment produced unexpected results because these are
> >>circular polarisers? Maybe, since the first will output circularly
> >>polarised light, I should not expect rotating the second to have any
> >>effect at all. This has just occurred to me, if I turn one around so
> >>that they are face to face, should I get the effect that I expect?
> >>Should I find a linear polariser or a pair of Polaroid sunglasses and
> >>put it in front of one of my filters to test it?
>
> >>Of course, another test is to put the filters on the camera and go and
> >>take some pictures. The problem with that is that the weather is
> >>lousy here and it is raining as you may expect for November rather
> >>than late May.
>
> >>--
> >>Seán O'Leathlóbhair
>
> >>Hi,
> >>A circular polarizer has a quarter wave retarder behind the main polarizer
> >>to circularly polarize the light coming off the regular polarizer. It's
> >>purpose is to allow proper exposure on cameras that send the light through a
> >>partially silvered mirror as many AF cameras do. The quarter wave retarder
> >>causes the polarizer not to seem to look right when peering through a couple
> >>of them. Place the FRONTs of the to polarizers facing each other and turn
> >>the. You should see the polarizers work as they should when you turn one.
>
> > <snip>
>
> > Yes, if I turn one polariser around, they behave as expected: when
> > lined up, they look pale grey, the same as one by itself; when one is
> > rotated 90 degrees, they go very dark, not completely black but a very
> > deep purple.
>
> > For a laugh, I tried them back to back, there is no noticeable
> > difference at all as you turn one. Considering the circular
> > polarising layer this makes sense.
>
> > The effect in the original orientation is still a puzzle: a change in
> > colour not density.
>
> > Anyway, I am happy now that I have two functioning polarisers.
>
> > It has stopped raining so I will pop out for a few photos before
> > dinner.
>
> > --
> > Seán O'Leathlóbhair
>
> Circular polarizers are not perfectly achromatic. That is, the
> quarter-wave plate is only a quarter-wave plate at one wavelength,
> unless they use a very expensive achromatic quarter-wave plate, which I
> doubt. (I've had them made for my research and paid $1000s.) So the
> linear-to-circular conversion is only "perfect" at one wavelength, and
> as you depart from that wavelength, some residual linear polarization
> will remain. Even the purely linear polarizers are not completely
> achromatic (unless they are very expensive) and will not give complete
> cancelation when crossed for all wavelegths; often blue light leaks
> through, though it can be red. The result is that, if you cross
> polarizers of various kinds in various ways, you can get some funny
> color effects. Two successive circular polarizers can give these color
> effects for these reasons. The first one will "leak" some
> wavlength-dependent linear polarized light that will interact with the
> second polarizer to various degrees, depending on its orientation.
>
> I hope this helps.


Yes, it does thanks though I think that I will read a bit more to
revise my own old A level (*) physics. Partly for fun but also
because a niece that lives with us is studying A level physics so
learning this will be useful for support. My son will probably follow
in a couple of years. (*) A levels are the exams that we study for
between 16 and 18 and hence immediately before university. A typical
good student does 4 or maybe 5 subjects at this level. I think that
polarisation of light comes up, it did in my day. She was studying
lenses recently: 1/u + 1/f = 1/ v etc.

Once, I turned one polariser around, I was quite impressed by them.
One by itself is as close to neutral grey as I can tell. Two in a row
when aligned appear the same as far as I can tell. When crossed, they
are not totally black but only very bright objects are visible such as
light bulbs. These appear a deep purple so I guess that the highest
wave lengths are leaking though.

Talking of the high wave lengths leaking through, is there any point
in stacking the polariser on top of the UV filter? I read that
digital sensors are not sensitive to UV so I guess not and the UV
would lose its protective value in this case. How about with film:
polariser alone or with UV filter? (The film question is just a
curiosity and not important)

--
Seán O'Leathlóbhair
 >> Stay informed about: Playing with polarisers 

On 30 May, 01:37, "Dave" <pleasereplytonewsgr....DeleteThis@shaw.ca> wrote:
> The practical use of polarizing filters is empirical, depending on what you
> see through your SLR rather than where little marks on the ring are.

Yes, of course, but I like to understand the theory behind what I do.
So, I expect to adjust the filter as you describe but I would like to
check the orientation afterwards.

Most of the shots in which I expect to use the polariser will not be
hurried so I plan to bracket the angle. How sensitive is the angle?
Would you recommend this bracketing and, if so, how many shots and
what (approximate) angle step?

> The big
> initial surprise will be that the degree of polarization varies depending on
> the angle of the camera vs the position of the sun. This will be a big
> problem for really wide angle shots, where polariaing filters don't work so
> well, because of an ugly variance of darkness across the sky. You can
> estimate the area of the sky most prone to dramatic polariaing effects. It
> is 90 deg off the position of the sun. To find that spot, point your index
> finger at the sun, and your thumb will be pointing to the area of the sky
> that will be most affected by polariaing filters. Of course it is a circular
> line across the sky, partially obscured by the earth that gets in the way.

That will be a problem for me. One type of photo that I have some
success with is extreme wide angle (up to 360) created by stitching
multiple shots together. I have already learnt things such as: use
manual exposure, focus, and raw conversion to get seamless joins. I
guess that I will have to omit the polariser for these shots. Maybe I
need a neutral density filter to darken the sky. That will have to
wait, I need to get some good shots with the polariser to convince the
wife that they were worth buying.

--
Seán O'Leathlóbhair
 >> Stay informed about: Playing with polarisers 

On 30 May, 22:39, Joseph Miller <mil....RemoveThis@ucolick.org> wrote:
> Seán O'Leathlóbhair wrote:
> > JohnR66 wrote:
>
> >>"Seán O'Leathlóbhair" <jwlaw....RemoveThis@yahoo.com> wrote in message
> >>news:1180446046.895814.242270@q75g2000hsh.googlegroups.com...
> >>I just bought a couple of circular polarising filters. Both are
> >>Hoya. I wanted one to improve my landscape photos. The reason for
> >>two is that I was dithering as to which lens to buy it for when the
> >>guy in the shop offered me a good discount for the two. My camera is
> >>a Canon 300D. The lenses being considered for polarisers were the:
> >>18-55mm kit lens and the 28-135mmm IS lens. The 28-135 should be the
> >>better lens but I am quite happy with the kit lens and I like the
> >>extra width for landscapes. Anyway, problem solved, I now have both
> >>sizes. One will also fit my 75-300mm but it would rather dark at the
> >>300 end so that may not be very useful.
>
> >>Question 1, the instructions on the box are minimal. There is an arrow
> >>on the rotating ring. When this is at the top does it mean that
> >>vertical polarisation is selected?
>
> >>Since I had two filters, I took them out of their boxes and looked
> >>through them. As I rotate them, they do change (so it seems that they
> >>really are polarisers) but not quite as I expected. The change seems
> >>to be more of colour than density. When lined up the filters appear
> >>slightly orangey and when 90 degrees apart they appear grey but not as
> >>dark as I hoped. If they were perfect, I would expect black. I guess
> >>that is hoping too much but I did expect a darker grey.
>
> >>Has this experiment produced unexpected results because these are
> >>circular polarisers? Maybe, since the first will output circularly
> >>polarised light, I should not expect rotating the second to have any
> >>effect at all. This has just occurred to me, if I turn one around so
> >>that they are face to face, should I get the effect that I expect?
> >>Should I find a linear polariser or a pair of Polaroid sunglasses and
> >>put it in front of one of my filters to test it?
>
> >>Of course, another test is to put the filters on the camera and go and
> >>take some pictures. The problem with that is that the weather is
> >>lousy here and it is raining as you may expect for November rather
> >>than late May.
>
> >>--
> >>Seán O'Leathlóbhair
>
> >>Hi,
> >>A circular polarizer has a quarter wave retarder behind the main polarizer
> >>to circularly polarize the light coming off the regular polarizer. It's
> >>purpose is to allow proper exposure on cameras that send the light through a
> >>partially silvered mirror as many AF cameras do. The quarter wave retarder
> >>causes the polarizer not to seem to look right when peering through a couple
> >>of them. Place the FRONTs of the to polarizers facing each other and turn
> >>the. You should see the polarizers work as they should when you turn one.
>
> > <snip>
>
> > Yes, if I turn one polariser around, they behave as expected: when
> > lined up, they look pale grey, the same as one by itself; when one is
> > rotated 90 degrees, they go very dark, not completely black but a very
> > deep purple.
>
> > For a laugh, I tried them back to back, there is no noticeable
> > difference at all as you turn one. Considering the circular
> > polarising layer this makes sense.
>
> > The effect in the original orientation is still a puzzle: a change in
> > colour not density.
>
> > Anyway, I am happy now that I have two functioning polarisers.
>
> > It has stopped raining so I will pop out for a few photos before
> > dinner.
>
> > --
> > Seán O'Leathlóbhair
>
> Circular polarizers are not perfectly achromatic. That is, the
> quarter-wave plate is only a quarter-wave plate at one wavelength,
> unless they use a very expensive achromatic quarter-wave plate, which I
> doubt. (I've had them made for my research and paid $1000s.) So the
> linear-to-circular conversion is only "perfect" at one wavelength, and
> as you depart from that wavelength, some residual linear polarization
> will remain. Even the purely linear polarizers are not completely
> achromatic (unless they are very expensive) and will not give complete
> cancelation when crossed for all wavelegths; often blue light leaks
> through, though it can be red. The result is that, if you cross
> polarizers of various kinds in various ways, you can get some funny
> color effects. Two successive circular polarizers can give these color
> effects for these reasons. The first one will "leak" some
> wavlength-dependent linear polarized light that will interact with the
> second polarizer to various degrees, depending on its orientation.
>
> I hope this helps.


Yes, it does thanks though I think that I will read a bit more to
revise my own old A level (*) physics. Partly for fun but also
because a niece that lives with us is studying A level physics so
learning this will be useful for support. My son will probably follow
in a couple of years. (*) A levels are the exams that we study for
between 16 and 18 and hence immediately before university. A typical
good student does 4 or maybe 5 subjects at this level. I think that
polarisation of light comes up, it did in my day. She was studying
lenses recently: 1/u + 1/f = 1/ v etc.

Once, I turned one polariser around, I was quite impressed by them.
One by itself is as close to neutral grey as I can tell. Two in a row
when aligned appear the same as far as I can tell. When crossed, they
are not totally black but only very bright objects are visible such as
light bulbs. These appear a deep purple so I guess that the highest
frequencies are leaking though.

Talking of the high frequencies leaking through, is there any point in
stacking the polariser on top of the UV filter? I read that digital
sensors are not sensitive to UV so I guess not and the UV would lose
its protective value in this case. How about with film: polariser
alone or with UV filter? (The film question is just a curiosity and
not important)

--
Seán O'Leathlóbhair
 >> Stay informed about: Playing with polarisers 

On 30 May, 03:04, nob... RemoveThis @noplace.org wrote:
> On 29 May 2007 10:52:14 -0700, Seán O'Leathlóbhair <jwlaw... RemoveThis @yahoo.com> wrote:
>
> >Yes, if I turn onepolariseraround, they behave as expected: when
> >lined up, they look pale grey, the same as one by itself; when one is
> >rotated 90 degrees, they go very dark, not completely black but a very
> >deep purple.
>
> >For a laugh, I tried them back to back, there is no noticeable
> >difference at all as you turn one. Considering the circular
> >polarising layer this makes sense.
>
> >The effect in the original orientation is still a puzzle: a change in
> >colour not density.
>
> You have stumbled on what filter makers stumbled on years ago. Now add in a
> layer of clear plastic behind the circular polarizer and add a linear polarizer
> behind that. You can then dial-in whatever color changes you want this way. Try
> various cellophanes (cigarette-pack wrapper plastics give you an intense blue to
> vivid orange option) or other perfectly clear packaging plastics that twist the
> wavelength. You'll have the same as those over-priced variable-color filters.
> Depending on the chosen clear plastic

Sounds fun, I will try it at the weekend.

> If you want to use 2 polarizers as a variable ND filter, then the front one will
> have to be a linear polarizer and the back one (camera side) the circular one.
> Or just use 2 linear polarizers.

Not a plan. I was just playing and testing the polarisers.

> BTW: Some of the most expensive polarizers are actually the worst in being able
> to filter out polarized light sources (reflections from flat surfaces). I tested
> some brands a few years ago. Tiffen's being the absolute worst. I have a $15
> generic that beats out a $60 Tiffen. To test them have a polarizer of good
> quality, meaning if 2 of them are placed at 90 degrees to one another nearly all
> bright light is extinguished completely and evenly across the faces of the
> filters. Then test other polarizers against one of them. You'll find that very
> few of the high-priced polarizers are able to extinguish the light adequately.
> Even worse is that most of them have the polarizing layer laid down unevenly,
> revealing strong banding and blotches in them when turned 90-degrees to a
> polarizer of known good quality.

These ones managed a very deep purple when crossed. It is a bit
harder to check how even it is since only bright objects (e.g. bulbs)
are visible.

> I suppose they get away with it because most people don't think to test them
> this simple way. They just assume if it costs more it must be higher quality.
> When in reality nothing is further from the truth these days.

I got these from a reasonably good price (by UK standards).

--
Seán O'Leathlóbhair
 >> Stay informed about: Playing with polarisers 

This message is not archived
 >> Stay informed about: Playing with polarisers 

And lo, Seán O'Leathlóbhair <jwlawler DeleteThis @yahoo.com> emerged from the ether
and spake thus:
> On 30 May, 22:39, Joseph Miller <mil... DeleteThis @ucolick.org> wrote:
>> Seán O'Leathlóbhair wrote:
>> > JohnR66 wrote:
>>
>> >>"Seán O'Leathlóbhair" <jwlaw... DeleteThis @yahoo.com> wrote in message
>> >>news:1180446046.895814.242270@q75g2000hsh.googlegroups.com...
>> >>I just bought a couple of circular polarising filters. Both are
>> >>Hoya. I wanted one to improve my landscape photos. The reason for
>> >>two is that I was dithering as to which lens to buy it for when the
>> >>guy in the shop offered me a good discount for the two. My camera is
>> >>a Canon 300D. The lenses being considered for polarisers were the:
>> >>18-55mm kit lens and the 28-135mmm IS lens. The 28-135 should be the
>> >>better lens but I am quite happy with the kit lens and I like the
>> >>extra width for landscapes. Anyway, problem solved, I now have both
>> >>sizes. One will also fit my 75-300mm but it would rather dark at the
>> >>300 end so that may not be very useful.
>>
>> >>Question 1, the instructions on the box are minimal. There is an arrow
>> >>on the rotating ring. When this is at the top does it mean that
>> >>vertical polarisation is selected?
>>
>> >>Since I had two filters, I took them out of their boxes and looked
>> >>through them. As I rotate them, they do change (so it seems that they
>> >>really are polarisers) but not quite as I expected. The change seems
>> >>to be more of colour than density. When lined up the filters appear
>> >>slightly orangey and when 90 degrees apart they appear grey but not as
>> >>dark as I hoped. If they were perfect, I would expect black. I guess
>> >>that is hoping too much but I did expect a darker grey.
>>
>> >>Has this experiment produced unexpected results because these are
>> >>circular polarisers? Maybe, since the first will output circularly
>> >>polarised light, I should not expect rotating the second to have any
>> >>effect at all. This has just occurred to me, if I turn one around so
>> >>that they are face to face, should I get the effect that I expect?
>> >>Should I find a linear polariser or a pair of Polaroid sunglasses and
>> >>put it in front of one of my filters to test it?
>>
>> >>Of course, another test is to put the filters on the camera and go and
>> >>take some pictures. The problem with that is that the weather is
>> >>lousy here and it is raining as you may expect for November rather
>> >>than late May.
>>
>> >>--
>> >>Seán O'Leathlóbhair
>>
>> >>Hi,
>> >>A circular polarizer has a quarter wave retarder behind the main polarizer
>> >>to circularly polarize the light coming off the regular polarizer. It's
>> >>purpose is to allow proper exposure on cameras that send the light through a
>> >>partially silvered mirror as many AF cameras do. The quarter wave retarder
>> >>causes the polarizer not to seem to look right when peering through a couple
>> >>of them. Place the FRONTs of the to polarizers facing each other and turn
>> >>the. You should see the polarizers work as they should when you turn one.
>>
>> > <snip>
>>
>> > Yes, if I turn one polariser around, they behave as expected: when
>> > lined up, they look pale grey, the same as one by itself; when one is
>> > rotated 90 degrees, they go very dark, not completely black but a very
>> > deep purple.
>>
>> > For a laugh, I tried them back to back, there is no noticeable
>> > difference at all as you turn one. Considering the circular
>> > polarising layer this makes sense.
>>
>> > The effect in the original orientation is still a puzzle: a change in
>> > colour not density.
>>
>> > Anyway, I am happy now that I have two functioning polarisers.
>>
>> > It has stopped raining so I will pop out for a few photos before
>> > dinner.
>>
>> > --
>> > Seán O'Leathlóbhair
>>
>> Circular polarizers are not perfectly achromatic. That is, the
>> quarter-wave plate is only a quarter-wave plate at one wavelength,
>> unless they use a very expensive achromatic quarter-wave plate, which I
>> doubt. (I've had them made for my research and paid $1000s.) So the
>> linear-to-circular conversion is only "perfect" at one wavelength, and
>> as you depart from that wavelength, some residual linear polarization
>> will remain. Even the purely linear polarizers are not completely
>> achromatic (unless they are very expensive) and will not give complete
>> cancelation when crossed for all wavelegths; often blue light leaks
>> through, though it can be red. The result is that, if you cross
>> polarizers of various kinds in various ways, you can get some funny
>> color effects. Two successive circular polarizers can give these color
>> effects for these reasons. The first one will "leak" some
>> wavlength-dependent linear polarized light that will interact with the
>> second polarizer to various degrees, depending on its orientation.
>>
>> I hope this helps.
>
>
> Yes, it does thanks though I think that I will read a bit more to
> revise my own old A level (*) physics. Partly for fun but also
> because a niece that lives with us is studying A level physics so
> learning this will be useful for support. My son will probably follow
> in a couple of years. (*) A levels are the exams that we study for
> between 16 and 18 and hence immediately before university. A typical
> good student does 4 or maybe 5 subjects at this level. I think that
> polarisation of light comes up, it did in my day. She was studying
> lenses recently: 1/u + 1/f = 1/ v etc.
>
> Once, I turned one polariser around, I was quite impressed by them.
> One by itself is as close to neutral grey as I can tell. Two in a row
> when aligned appear the same as far as I can tell. When crossed, they
> are not totally black but only very bright objects are visible such as
> light bulbs. These appear a deep purple so I guess that the highest
> wave lengths are leaking though.
>
> Talking of the high wave lengths leaking through, is there any point
> in stacking the polariser on top of the UV filter? I read that
> digital sensors are not sensitive to UV so I guess not and the UV
> would lose its protective value in this case. How about with film:
> polariser alone or with UV filter? (The film question is just a
> curiosity and not important)

Somewhat OT, but I heard a story about a professional stock shooter
who, when asked his opinion about using UV filters as a protective
measure, he said, "UV filters? I don't use LENS CAPS. It takes too
much TIME to take them OFF."

He shoves everything into his bag without UV filters OR lens caps,
just bare elements into the bag. Considering he was carrying around a
Canon 500mm L, I think he is somewhat successful. To each their own!
At least he put the 500mm into the individual case it came with...

--
Aaron
http://www.fisheyegallery.com
http://www.singleservingphoto.com
 >> Stay informed about: Playing with polarisers 

Seán O'Leathlóbhair wrote:

>>Circular polarizers are not perfectly achromatic. That is, the
>>quarter-wave plate is only a quarter-wave plate at one wavelength,
>>unless they use a very expensive achromatic quarter-wave plate, which I
>>doubt. (I've had them made for my research and paid $1000s.) So the
>>linear-to-circular conversion is only "perfect" at one wavelength, and
>>as you depart from that wavelength, some residual linear polarization
>>will remain. Even the purely linear polarizers are not completely
>>achromatic (unless they are very expensive) and will not give complete
>>cancelation when crossed for all wavelegths; often blue light leaks
>>through, though it can be red. The result is that, if you cross
>>polarizers of various kinds in various ways, you can get some funny
>>color effects. Two successive circular polarizers can give these color
>>effects for these reasons. The first one will "leak" some
>>wavlength-dependent linear polarized light that will interact with the
>>second polarizer to various degrees, depending on its orientation.
>>
>>I hope this helps.
>

<snip>

> Once, I turned one polariser around, I was quite impressed by them.
> One by itself is as close to neutral grey as I can tell. Two in a row
> when aligned appear the same as far as I can tell. When crossed, they
> are not totally black but only very bright objects are visible such as
> light bulbs. These appear a deep purple so I guess that the highest
> wave lengths are leaking though.

Actually it is the other way around. In the visual wavelengths
(blue-green through near-red), the polarizer is extremely good,
effectively 100%, so crossing them gets rid of all that light. But at
the shortest wavelengths the filter is less than 100% effective, so that
crossing two of them will still transmit a bit of unpolarized purple
light. Probably there is a bit of red light leaking through, too, as
these filters often lose their effectiveness somewhere in the red.
>
> Talking of the high wave lengths leaking through, is there any point
> in stacking the polariser on top of the UV filter? I read that
> digital sensors are not sensitive to UV so I guess not and the UV
> would lose its protective value in this case. How about with film:
> polariser alone or with UV filter? (The film question is just a
> curiosity and not important)

There is no need for the UV filter, both because the sensor won't see it
and quite often the polaroid filter is a poor transmitter of UV light
itself. If I'm lazy, I don't remove my UV filter when I use a polaroid
filter, but I should. There's no point in going through an extra piece
of glass with two air-glass boundaries unless you have to. The extra
glass can reduce contrast unless it and the polaroid filter are very
well coated. As for film, it depends on the polarizer and the camera.
Many film cameras don't benefit significantly from a UV filter (the
glass used for camera lenses really doesn't transmit well in the UV),
though some do, and as I said, the polarizer itself can often be a good
UV filter.

Joe
 >> Stay informed about: Playing with polarisers