Polarising a flash?

chris321

chris321

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O.K, while playing with my polariser just now, I started thinking (always dangerous :D ), and I was just wondering what would happen if you had a polariser on the lens, and also in front of the flash? I only have one, so can't try this out, but would it be possible to selectively illuminate parts of an image, depending on what angle both of the polarisers were at, and the angles of the surfaces.

I'm trying to work it out in my head, but I just get confused, so I was wondering if anyone had tried this?

Yours confusedly, :D

Chris
 
But the light would probably be scatted when it is reflected off stuff, so the net result would be, as flash says, impercievable except for a loss of flash power.
 
Using a polarising filter on the lens and polarising film on your strobes is a common way of cutting out glare when photographing oil paintings.
 
steveb said:
Using a polarising filter on the lens and polarising film on your strobes is a common way of cutting out glare when photographing oil paintings.
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True, for for photographing many other subjects too. Unlike sunlight, the light from flash is not naturally polarised so using a polarising screen just on the lens only has a limited effect.

The polarising screen gels used on flash heads are easily damaged by heat and not cheap, but are a standard tool in pro studios because using them in conjunction with a polarising screen on the lens is the only way to get the job done sometimes
 
theoretically you could lose all light from a given flash head that way as waves cant pass through 2 perp. polarisers hmmm.

Gary can you explain why it reduces glare? Is it because the reflected light is polarised differently to the rest of the light so can be removed?

Intrigued by this concept
 
Ah right, sounds like I sort of had the right idea, but was over-simplifying it a bit. Thanks a lot guys!

Chris
 
david1701 said:
theoretically you could lose all light from a given flash head that way as waves cant pass through 2 perp. polarisers hmmm.

Gary can you explain why it reduces glare? Is it because the reflected light is polarised differently to the rest of the light so can be removed?

Intrigued by this concept
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It will reduce glare because the light will all be at the same angle rather than at infinite angles so it can only be reflected back to the lens by one particular angled surface rather than many.

Garry, how is the sunlight naturally polarised? In the physics I did through college/uni I understood that light is emmitted at all angles from most light sources. Using a polariser on the flash will reduce the angles that light is emitted and therefore reduces the reflection possibilitied, but then as soon as it gets reflected from an object it will no longer be polarised and will require a polariser on the lens to get it polarised before hitting the sensor/film.

You can do an experiment (the results of which depend on the quality of the polarisers) by having two polarisers aligned you should beable to see clearly, turn one to 90 degrees and you should be able to see nothing. Try doing the same pointing directly into a flash/sun (be careful of your eyes!) The same will be true.

If the sun's light was polarised, you could just use one polariser and look directly at the sun and you should beable to make it completely dark and the same way that you did with the two polarisers. I bet that you can't because the sun's light isn't polarised however I'd be interested to know if I'm wrong.
 
CraigDouglas said:
It will reduce glare because the light will all be at the same angle rather than at infinite angles so it can only be reflected back to the lens by one particular angled surface rather than many.

Garry, how is the sunlight naturally polarised? In the physics I did through college/uni I understood that light is emmitted at all angles from most light sources. Using a polariser on the flash will reduce the angles that light is emitted and therefore reduces the reflection possibilitied, but then as soon as it gets reflected from an object it will no longer be polarised and will require a polariser on the lens to get it polarised before hitting the sensor/film.

You can do an experiment (the results of which depend on the quality of the polarisers) by having two polarisers aligned you should beable to see clearly, turn one to 90 degrees and you should be able to see nothing. Try doing the same pointing directly into a flash/sun (be careful of your eyes!) The same will be true.

If the sun's light was polarised, you could just use one polariser and look directly at the sun and you should beable to make it completely dark and the same way that you did with the two polarisers. I bet that you can't because the sun's light isn't polarised however I'd be interested to know if I'm wrong.
Click to expand...
Well, in theory sunlight itself isn't naturally polarised, you're completely right about that.
But in practice it is, because the light becomes scattered during its 93 million mile journey (pollution, reflection from clouds etc) and scattered light IS polarised.
 
If you put a polarising sheet over a lightbox (same as a flash head....) and one on the lens, it is possible to have a backlit object on a black background - you can polarise the light in such a way that the lightbox goes black, because the filters block the lot....but not what is reflected off th eedges of the subject (or through it).
 
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