Question on Circular polarizer and rotating lens element

Hi,
If I am using a circular polarizer on a digital SLR, I heard that a
rotating lens element will rotate the polarizer and it will basically
make it useless. As I understand optics, a circular polarizer
basically changes the phase between the electrical and magnetic
field.

1. Am I right in saying that a cirucular polarizer is basically a
linerar polarizer + a quarter wave (QW) plate.
2. So, its the rotation of the linear polarizer element which causes
the problem, since it can either polarize or not-polarize (according
to the orientation).

Any suggestions, links, etc.?

Thanks.

PGPS wrote:
Hi,
If I am using a circular polarizer on a digital SLR, I heard that a
rotating lens element will rotate the polarizer and it will basically
make it useless. As I understand optics, a circular polarizer
basically changes the phase between the electrical and magnetic
field.

1. Am I right in saying that a cirucular polarizer is basically a
linerar polarizer + a quarter wave (QW) plate.
2. So, its the rotation of the linear polarizer element which causes
the problem, since it can either polarize or not-polarize (according
to the orientation).

Any suggestions, links, etc.?



Basically what you say is true. Note that the actual polarizer element
faces to the subject and the quarter wave place is toward the camera.

If the lens front rotates, you must either rotate the polarizer
to the correct position after it stops, or, if that is impossible,
simply use your third hand to hold the polarizer in front of the
lens, not attached to it. This works if you have even a modestly
steady hand.

Doug McDonald

In article . com,
PGPS wrote:

Hi,
If I am using a circular polarizer on a digital SLR, I heard that a
rotating lens element will rotate the polarizer and it will basically
make it useless. As I understand optics, a circular polarizer
basically changes the phase between the electrical and magnetic
field.

1. Am I right in saying that a cirucular polarizer is basically a
linerar polarizer + a quarter wave (QW) plate.
2. So, its the rotation of the linear polarizer element which causes
the problem, since it can either polarize or not-polarize (according
to the orientation).

Any suggestions, links, etc.?

Thanks.

Enter Circular polarizer into Google. Lots of information available.

--
Es ist nichts schrecklicher als eine tätige Unwissenheit.

Johann Wolfgang von Goethe

An easy way to look at this is look through the filter as the camera
would and rotate it, you will see the normal effects, now look through
it backwards and you should not see any effects of polarization.

It is likely you will still see an affect looking through the filter
backwards, but it depends on the details of the scene. Suppose the angle
of the polarized light from the scene lines up with the fast axis of the
quarter-wave plate. Then that plate won't do anything to the poloarized
light, but the linear polaroid behind it will diminish its intensity;
it's at a different angle. So as you rotate the filter you will see some
effect as the angle of the plate and polaroid interact with the scene
polarization. This is a minor point, but I didn't want someone to think
their filter was faulty if they tried this and saw an effect on the
light. Take two circular polarizers or one circular and one linear and
change their order or flip them and you can get some interesting,
different effects as you rotate them.

Joe

In article . com,
PGPS wrote:

Hi,
If I am using a circular polarizer on a digital SLR, I heard that a
rotating lens element will rotate the polarizer and it will basically
make it useless. As I understand optics, a circular polarizer
basically changes the phase between the electrical and magnetic
field.

1. Am I right in saying that a cirucular polarizer is basically a
linerar polarizer + a quarter wave (QW) plate.
2. So, its the rotation of the linear polarizer element which causes
the problem, since it can either polarize or not-polarize (according
to the orientation).

Any suggestions, links, etc.?

Thanks.

Points (1) and (2) are correct. The light first goes through the linear
polaroid. It does all the work on the light that you wish to have
happen. The quarter-wave plate, which converts the linear to circular
polarization is there for the benefit of downstream camera elements. So,
if your lens rotates the filter, you must re-set the filter before
shooting to the angle you want. There's no way around that.

Joe

On 21 Sep 2007 19:19:09 GMT, Marty Fremen wrote:

Probably precisely because of that problem, not many lenses rotate their
front element / filter mount when focussing or zooming. They used to maybe
30+ years ago but I've not had any cameras or lenses in the last 25 years
which suffered a rotating front element. Even my point and shoot digital
doesn't (not that it has a filter thread anyhow). I don't have a DSLR
though - maybe there's been some regression in lens design since DSLRs came
in?

There has indeed been such a regression. The Nikon D40 DLSR kit lens has a
rotating front element. PITA.

If you do have such a lens, it's not a huge problem, since the filter
will freely rotates in its mount whilst attaced to the camera. Either take
hold of it in one hand whilst focussing/zooming to stop it rotating, or
else correct its orientation afterwards.

Because of the slop in the focusing mechanism, you really need to...

1. focus
2. adjust polarizer
3. remove fingers from polarizer
4. focus again.

This is a significant problem if you need to take fast shots.