Polarizer on a M

[lars_bergquist]

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You are right cost more but can also adjust how much of a affect I want. I guess it amounts to choices.

 

Cheers Jan

Linear and circular polarisers do not differ at all in their effect on the subject, and how you regulate it, viz. by turning it. The difference is that a circular polariser has a second layer behind the polariser itself, a layer that de-polarises the light that has passed the polariser. I have experimented with both kinds of polarisers on M cameras and on traditional SLR cameras, without a secondary mirror behind a semi-silvered patch on the main reflex mirror (this is where exposure problems can arise) and there is not the slightest difference.

 

The fact that the light *behind* the polariser is de-polarised again does not mean that the image changes. Light that was stopped or attenuated by the polariser cannot be magically re-created inside the camera, just as light that has been stopped by a front cap cannot be resurrected ...

 

Many photographers, who are perhaps not very well versed in the physical sciences, seem to have got the vague idea that circular polarisers are the way to go because they are more 'hi-tech'. They deceive themselves, often with some friendly pushing by the salesmen.

 

The old man from the Age of Edwin Land

[lars_bergquist]

I learned a very simple, but accurate, method for using a polarizer on a rangefinder camera from Niels Thorsen (if I haven't mis-remembered, or misspelled his name) of Leica NJ.

 

While metering through the lens with the polarizer attached, turn the outer polarizer ring and notice the point at which the meter indicates that the least amount of light is getting through. This will be the point of maximum polarization. After you decide how much polarization you want, re-adjust f-stop or shutter speed, if necessary, for correct exposure. Brent

While this advice is correct in theory, practice is different, for two salient reasons.

 

First, it presupposes, in order to have a significant effect, that the amount of polarised light that you want to attenuate is great in relation to the total amount. This is true if you have a large expanse of blue sky, for instance. But if there are some white clouds about, it diminishes until your exposure readout will not notice much difference. And suppose that you are making a portrait, and taking out reflexes in the subject's spectacles. How much exposure difference would that make?

 

Second, an analog meter needle can react to very small changes. A digital display, which is what we have in every M since the M6TTL, both on auto and manual, reacts only in pretty coarse steps. This is quite useless for regulating anything but very drastic effects. So my advice is to learn what a polariser is, and what it does to light, and how light reflected from the subject can be polarised.

 

The old man from the Age of Edwin Land

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[lars_bergquist]

My only caution is All my Tiffins, linear and circ, impart a green cast. No leica or B+W impart a color cast except some very old Leicas made before color folm was popular.

Early polarisers were based on crystals of a polarising mieral called herapatite, which was not colourless. Edwin Land invented the modern 'Polaroid' polarising foil, in which are embedded needle-shaped crystals of an organic compound (related to or identical with quinine, by the way). These are then oriented in parallel by stretching the foil. The result is the neutral-grey polarisers we use. How Tiffen make their filters I do not know.

 

The old man from the Age of Edwin Land

[Rik]

Thanks for your comments on the Leicagoodies Steps.

 

As to the malaware issues with the website: I have had some major problems recently with this and am desperately trying to correct this, but it's not easy!

 

Leicagoodies is still in business and I hope to be able to get a 'clean' version on line of the website soon. In the mean time thanks to all the positive comments.

 

Rik Plomp

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[andalus]

One msg in this thread purports to tell one how to set a mark on the pol filter. How does this make sense? I have to screw the filter on to the lens. Does this not make the mar innacurate?? Am I really dumb about this.

 

Anyway, I have a Kaesemann B&W linear pol for my Lux 50 ASPH. I figure the best way to use is frame a shot, meter correctly, then tutn the pol until one sees a flicter or light in the left arrow in the viewfinder. At that point less light entering lens, so therefore that must be when polarizer is working the most. Does this make sense?

[farnz]

One msg in this thread purports to tell one how to set a mark on the pol filter. How does this make sense? I have to screw the filter on to the lens. Does this not make the mark innacurate?? ...

Most polarising filters are made up of a fixed, threaded ring that you screw into the filter thread on your lens, and a free-spinning ring - attached to the fixed ring - that holds the polarised glass. The mark gives a reference for setting the polarised glass so that the maximum polarising effect is achieved.

 

HTH,

Pete.

[lars_bergquist]

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Exactly, Pete. Of course you put the mark on the outher, rotating ring that has the actual filter mounted in it -- I should pehaps have mentioned that, but it seemed too obvious!

 

The old man from the Age of Edwin Land (of Polaroid)

Edited December 7, 2009 by lars_bergquist

[tgray]

Linear and circular polarisers do not differ at all in their effect on the subject, and how you regulate it, viz. by turning it. The difference is that a circular polariser has a second layer behind the polariser itself, a layer that de-polarises the light that has passed the polariser.

 

That's not exactly true. Circular polarizers have a quarter wave plate, converting the linearly polarized light to circularly polarized light. Circularly polarized light is NOT de-polarized.

[lars_bergquist]

That's not exactly true. Circular polarizers have a quarter wave plate, converting the linearly polarized light to circularly polarized light. Circularly polarized light is NOT de-polarized.

Yes of course. But first, 'circularly polarized light' is simply non-polarized light, light that has no dominant direction of polarization. That is just the light that arrives from Old Sol. What a polarizer does it to select the light that has its waves oriented in one particular direction, i.e. it stops light in the other directions. The quarter-plate then scrambles the remaing light, de-polarizing it again. -- In other words, what a polarizer does is simply to de-emphasize or block some of the light that is reflected from the subject. Both types do that equally. What happens then is immaterial, in principle.

 

Second, polarized light activates a sensor or a piece of film in exactly the way that un-polarized ('circularly polarized') light does. There is no 'pictorial' difference. Get this: (1) The effect on the image-making light is in the filter only, and (2) any difference behind the filter and lens occurs only if and where the light encounters and is passed through a *second* polarizing agent, such as a semi-silvered reflex mirror. And there is only one such mirror in a M, and that is the beamsplitter prism in the main finder. The light that passes this beamsplitter meets your own eye only, no sensor or film.

 

So, save your money and buy linear. -- The old man from the Age of Edwin Land

Edited December 11, 2009 by lars_bergquist

[tgray]

Yes of course. But first, 'circularly polarized light' is simply non-polarized light, light that has no dominant direction of polarization. That is just the light that arrives from Old Sol. What a polarizer does it to select the light that has its waves oriented in one particular direction, i.e. it stops light in the other directions. The quarter-plate then scrambles the remaing light, de-polarizing it again.

 

 

I agree with you that a linear polarizer is sufficient if you don't have an autofocus SLR, but circularly polarized light IS NOT unpolarized. It's light where the plane of polarization rotates as it propagates.

 

Here's a link that talks about the different polarizations of light.

 

I know this is a nitpicky detail, but still.

[lars_bergquist]

Now we are down to very small nits indeed. By the way, not only SLR cameras with autofocus have semi-silvered mirrors. Some with manual focusing used one for reflecting light down to an exposure sensor in the mirror chamber. In this case, incorrect exposure could result, which was the original reason whu circular polarizers were introduced in photography.

 

The old man from the Age of Edwin Land

[tgray]

Indeed. Reflection off of any surface can partially or totally polarize light depending on the angle. If you have a mirror or semi-silvered mirror in your camera for any reason, using a linear polarizer with it could affect some of its functions. Circular polarizers are better choices there.