Beginner Using two polarising lens filters to block 100% light

techno79

techno79

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It's not entirely camera related but:
As I understand it, if you have two polarising filters stacked on top of each (aligned to the way they polarise) then they will block about 50% of the light. But as you rotate one of the filters it blocks more and more light until it blocks 100% light when it is 90 degrees. This is exactly what happens with two iMax 3D cinema glasses.

I needed to do this using something else so I purchased some cheap polarising camera lens filters from ebay but they do not exhibit the same behaviour. There is a subtle change of light but it does not block anywhere near 100% of the light. In fact, it doesn't seem to block any more light than the first filter. The only change is appearance of the colour of the combined lens which turns from a 50% tinted black to a 50% lightly brownish/black tint.

So my question is, is this normal for camera lenses or are these a problem with these specific ones I've purchased (they were the cheapest ones I could find).

Or put another way, using two "proper" polarising camera lenses aligned 90 degrees to each other, will they block 100% of the light?

TIA
 
There are two type of polarising filters that I am aware of, linear and circular - I suspect the effect you see (100% light blockage) will only happen with linear.
 
techno79 said:
It's not entirely camera related but:
As I understand it, if you have two polarising filters stacked on top of each (aligned to the way they polarise) then they will block about 50% of the light. But as you rotate one of the filters it blocks more and more light until it blocks 100% light when it is 90 degrees. This is exactly what happens with two iMax 3D cinema glasses.

Or put another way, using two "proper" polarising camera lenses aligned 90 degrees to each other, will they block 100% of the light?

TIA
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Not completely - they actually can only block light which is polarised - some light is not polarised so a portion of that light will always get through even though two filters aligned to theoretically block all light will always let some through, even though a much reduced amount.
 
I normally use a lens cap for 100% light blockage :)
Arent Circular and Linera polarisers the same (essentially) just digital sensor are confused by Linera, hence we now have 2 versions?
Matt
 
GreenNinja67 said:
I still use an old linear polariser on one of my lenses on my AF digital DSLR.

Seems to work fine.
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Just did a bit of research and one reporter did exactly that because he had used circular filter on his digital camera and found af was "off", images were a bit soft but he wasnt sure if it was because he hadnt spent enough on the circular filter, I guess a cheap one might have that effect.
 
I used an old laptop screen. Apparently the light from them is polaried. Set it to be pure white.
I put a polariser on my camera and when positioned correctly everything went black.
Then I put some plastic cutlery on the laptop screen. Because this is pressed plastic it sets up stress lines in the plastic. This plays havoc with the light patterns and the polarising effects go haywire. This is the shot>>>>

Fork handles by Frank Yates2010, on Flickr

Just a bit of fun on a cold wet afternoon!
 
Possible Matt,

I have a 52mm linear PL and a 77mm CPL, both made by Hoya and I'm unable to see a difference in their use.
 
Thanks Terry. Can't claim any originality - I saw the technique somewhere on 't interweb.
 
GreenNinja67 said:
I have a 52mm linear PL and a 77mm CPL, both made by Hoya and I'm unable to see a difference in their use.
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Do you have problems using AF with a linear PL?

techno79 said:
As I understand it, if you have two polarising filters stacked on top of each (aligned to the way they polarise) then they will block about 50% of the light. But as you rotate one of the filters it blocks more and more light until it blocks 100% light when it is 90 degrees. This is exactly what happens with two iMax 3D cinema glasses.
Click to expand...


Thats the way variable ND filters work.
 
AndyG123 said:
Can someone explain why you would want to block 100% light from a camera?
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Not quite 100% but a 10 stop ND reduces light transmission by 1000x times and are much used by photographers. You may also wish to block much of the light transmission for various industrial photographs. Wielding being one such use
 
So are polarizing camera lens filters meant to be circular polarising or linear polarizing or both? The ones I bought are probably circular.
 
techno79 said:
So are polarizing camera lens filters meant to be circular polarising or linear polarizing or both? The ones I bought are probably circular.
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Either. Linear used to interfere with af systems iirc - whether they do or not now I don't know as I only use a circular polariser ...
 
techno79 said:
So are polarizing camera lens filters meant to be circular polarising or linear polarizing or both? The ones I bought are probably circular.
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Almost all polarising filters for popular use are circular these days. You're unlikely to find anything else in the average camera shop, but linear polarisers are available, eg from B+W and others. Linear polarisers are supposed to interfere with the AF and/or metering of some cameras, though a few quick tests on my Canons hasn't shown this. Anyway, we have circular polarisers that do exactly the same job with zero penalty, so why not. A circular polariser is just a linear polariser with a quarter-wave plate on the back which circularises or 'de-polarises' the light.

To block 100% of the light you need a linear polariser in front with either a linear or circular polariser behind, rotated 90 degrees. This will block close to 100%, and the light source doesn't need to be polarised. Edit: if the light source is polarised, such as an LCD screen, then you only need one filter to block the light (see Mickledore's post) - either a circular or linear polariser.
 
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Try turning the front filter (closer to the light source) around so its facing backwards
 
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techno79 said:
So are polarizing camera lens filters meant to be circular polarising or linear polarizing or both? The ones I bought are probably circular.
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A circular polariser will only let through the light in one direction (say 50%) then randomly 'scatter' it again in all directions. A second polariser will then polarise 50% of that light.

You need the first polariser to be linear. Possibly the second too, depending on what you want to do.


Steve.
 
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I think 2 linear pola filters used together on digital may give you a blotchy effect, I seem to remember it was something to do with the sensor?
 
Steve Smith said:
A circular polariser will only let through the light in one direction (say 50%) then randomly 'scatter' it again in all directions. A second polariser will then polarise 50% of that light.

You need the first polariser to be linear. Possibly the second too, depending on what you want to do.


Steve.
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If you flips the first filter so it faces backwards it will act as a linear polarizer.
 
soeren said:
If you flips the first filter so it faces backwards it will act as a linear polarizer.
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But would it work as a polariser then? The light you would block previously being polarised in one direction would now be random.

I only have linear polarisers so I can’t check.


Steve.
 
Steve Smith said:
But would it work as a polariser then? The light you would block previously being polarised in one direction would now be random.

I only have linear polarisers so I can’t check.


Steve.
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Why shouldn't it? The light coming out of it will be polarized no matter how random it is entering it. The second filter should be oriented as intended.
 
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Because if it’s backwards, it would randomise it before polarising. So some light which would have been blocked will now pass due to its new orientation, likewise, some light which would have passed through will now be blocked.

EDIT: Unless you are just referring to its ability to work with another polariser to reduce the total light going through. In which case, I agree. It just won’t work as we expect a polariser to work.


Steve.
 
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A circular polariser when used back to front does nothing - no polarising effect. It simply acts as a mild neutral density filter. A linear polariser works either way around.
 
HoppyUK said:
A circular polariser when used back to front does nothing - no polarising effect. It simply acts as a mild neutral density filter. A linear polariser works either way around.
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Yes it polarizes light when flipped backwards but since it's circular any polarized light from reflections will be depolarized first and thus not "turned off. So the light entering the second polarizer is polarized and can be turned off. Checked, works. Now i just need to get access to a PC to upload
 
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in advance sorry for really bad IQ

Circular polariser on camera and normally oriented circular polariser on table

polfilter skaleret.jpg


Now the circular polariser on the table is flipped over so the filter thread is facing down.
Now its doing what Techno79 is asking for

flippet polfilter skaleret.jpg

to summarize. the front of the (Circular polarising) filters must face each other in order to do what you asked for in your OP
 
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soeren said:
Yes it polarizes light when flipped backwards but since it's circular any polarized light from reflections will be depolarized first and thus not "turned off. So the light entering the second polarizer is polarized and can be turned off. Checked, works. Now i just need to get access to a PC to upload
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I think I misunderstood. I was referring to just one CPL used back to front. Apologies :)
 
HoppyUK said:
I think I misunderstood. I was referring to just one CPL used back to front. Apologies :)
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Thats only due to my lack of english vocabulary and not being able to explain :) back to front, think Ill remember that.
 
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A few years ago I was taking photos abroad with my Canon G11, I was wearing polarising sunglasses and when I positioned the camera in the portrait format the rear screen would go blank!! I thought there was a loose wire in the camera and was losing power to the screen when I turned it. Took me 2 days to realise what was happening. :)
 
Problem arises when adding a third polarizer to replicate the experiment in the video, this one must be linear
 
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