Question about Neutral Density filters

[Alex]

<BLOCKQUOTE><font size=2 face="Verdana, Helvetica, sans-serif">quote:</font><table border="0" width="90%" bgcolor="#333333" cellspacing="1" cellpadding="0"><tr><td width="100%"><table border="0" width="100%" cellspacing="0" cellpadding="2" bgcolor="#FF9900"><tr><td width="100%" bgcolor="#DDDDDD"><font size=2 face="Verdana, Helvetica, sans-serif">Originally posted by Actor:

Personally I would buy two: a 0.3ND and a 0.6ND. the former gives me 1 stop, the latter 2 stops and stacking them together gives 3 stops, enough for most situations. If you want more then get a 0.1ND and a O.2ND. With these four you can get anything from 0.1ND to 1.2ND is 0.1ND steps.

If you want to get a fifth it should be a 1.2ND</font></td></tr></table></td></tr></table></BLOCKQUOTE>

This makes sense, however, how often do you really need a .3 ND? Does anyone really ND for only one f-stop?

When used with a Polarizer a one f-stop ND (a .3) makes sense. I doubt I would ever use only one f-stop worth of ND by itself.

[Mike Buckles]

Nice Pics, S8booster, nice transfer too! This has been an interesting and informative discussion! Now I'm thinking a polarizing filter might be nice to have, too! Do they come in different strengths too, or is it just a standard filter? MIke

[TA152]

<font color="#a62a2a" size="1">[ August 28, 2003 11:07 PM: Message edited by: S8 Booster ]</font>

[Alex]

I just have an opinon on this. A plain polarizer should cause anywhere from 1/2 stop to 2 stops decrease in light, depending on how the polarizer is turned in relation to the angle of the sun. You should actually see the picture deepen in intensity as you turn it, as you keep turning it, it will then back off and brighten.

I think some Polarizer's might come with a built in ND filter, but I'm not sure.

I like the pictures, if you had done the film transfers on a rank cintel or equivalent, they would be just stunning pictures.

[Actor]

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A plain polarizer...
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Is there any other kind? Seriously, I don't know of any other kind.
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...should cause anywhere from 1/2 stop to 2 stops decrease in light...
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All polarizers have a filter factor of 2.5 which corresponds to 1 and 1/3 stops of attenuation.
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... , depending on how the polarizer is turned in relation to the angle of the sun.
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What do you mean by "angle of the sun"? My first take is that you mean the angle between the horizon and the center of the sun's disk, i.e., the angle measured by a navigator with a sextant. However, what this could possibly have to do with the function of a polarizer is beyond me so you probably mean something else, but I know not what.


I have more to say on this subject but I'll post this now and give Alex a chance to respond.

[Alex]

<BLOCKQUOTE><font size=2 face="Verdana, Helvetica, sans-serif">quote:</font><table border="0" width="90%" bgcolor="#333333" cellspacing="1" cellpadding="0"><tr><td width="100%"><table border="0" width="100%" cellspacing="0" cellpadding="2" bgcolor="#FF9900"><tr><td width="100%" bgcolor="#DDDDDD"><font size=2 face="Verdana, Helvetica, sans-serif"><font size=2 face="Verdana, Helvetica, sans-serif">
A plain polarizer...
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Is there any other kind? Seriously, I don't know of any other kind.

###### The other kind would be a polarizer with a built-in ND, I thought I have seen them for sale, but I'm not sure. #####

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...should cause anywhere from 1/2 stop to 2 stops decrease in light...
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All polarizers have a filter factor of 2.5 which corresponds to 1 and 1/3 stops of attenuation.

############# Perhaps you could explain what filter factor means? I just did a test with a polarizer and the auto-exposure indeed shifted between 1/2 to 1&1/2 f-stops as I rotated the polarizer, however, the filter itself also causes a loss of one f-stop just by putting it on the lens. (kind of like an ND factor of 3, no?)

Effectively meaning a total loss of 1.5 to 2.5 f-stops. Attenuation (meaning the actual variance of the polarizer) is not the only issue, one must also consider the loss of available light by merely putting the filter on the lens.

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... , depending on how the polarizer is turned in relation to the angle of the sun.

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What do you mean by "angle of the sun"? My first take is that you mean the angle between the horizon and the center of the sun's disk, i.e., the angle measured by a navigator with a sextant. However, what this could possibly have to do with the function of a polarizer is beyond me so you probably mean something else, but I know not what.

I have more to say on this subject but I'll post this now and give Alex a chance to respond.

######### I'm relating the angle of the sun as to where it is in the sky versus where you point the polarizer. In some parts of the sky you can get a deeper blue than other parts of the sky. I assume this is due to where the sun is positioned in the sky, however, it could just as well have to do with how blue the sky was to begin with in the various parts of the sky that the polarizer is pointed at. Pointing the camera in wide angle at the sun won't render as blue a sky. (and be careful not to look directly at the sun for more than a brief moment, this can cause eye damage) Through the viewfinder, the blue sky gets more intense as you turn the polarizer depending on the angle/location from the sun. #############

[cameraguy]

Linear polarizer (plain) and Circular polarizer (with shots). A circular is needed for AF and beam splitter cameras.

[Mike Buckles]

Alex, you have a Bell and Howell MS45 like me...what type of polarizing filter do you think it would take...?

[Actor]

I think we need to review some fundamentals here.

Let's begin with the nature of light. Light is a wave phenomenon. Never mind the quantum theory. Light is a wave. There are two kinds of waves: longitudinal and transverse.

Longitudinal waves are waves in which the action takes place along the direction in which the wave is propagated, i.e., traveling. Take a child's slinky toy, stretch it out and give it a nudge at one end. A wave will travel from one end to the other. This is a longitudinal wave. Sound waves are longitudinal waves.

Transverse waves are waves in which the action takes place in a direction perpendicular to the direction of the waves propgation. Drop a pebble in a pool of water and waves will travel outward from this disturbance. The action of the waves is vertical, perpendicular to the direction the wave is traveling.

Light waves are transverse waves.

Now the thing about transverse waves is that there is more than one direction in which the action of the waving can take place and still be perpendicular to the direction of propagation. In fact there are an infinite number from 0 to 360 degrees.

If a light wave is coming at you (or your camera) and it is waving vertically then it is said to be vertically polarized. If it is waving horizontally then it is horizontally polarized. But it could be waving at an angle of say 57 degrees it which case it is a combination of vertically and horizontally polarized.

Now the quantum theory raises again. Light from the sun is circularly (or randomly) polarized. This means that the light consists of billions of quanta per second and each quanta has its own polarization. The distribution is random, scattered at every angle from 0 to 360 degrees.

Circular polarization is characteristic of not only the sun but of most light sources.

However, while light may be circularly polarized at its source it can quickly become polarized. The best example is sunlight which is reflected off of water and becomes vertically polarized. Sailors and beach goers buy horizontally polarized sunglasses which block the vertically polarized glare while letting through the still randomly polarized light from most other objects.

Light reflected off smooth surfaces generally gets polarized. A photographer might use a polarizer to get rid of the horizontally polarized glare from the windows of a building.

A polarizer can be used to deliberately polarize light. If two polarizers are stacked and each set to zero degrees then the light will still get through, attenuated 1 1/3 stops of course. If the second polarizer is set at 90 degrees then no light will get through.

This was the basis of color 3D movies in the 1950s. The movie was projected from two projectors. One was fitted with a horizontal polarizer and the other with a vertcal polarizer. Audiences were given cheap polarized 3D glasses. It was hoped that eventually audiences would buy their own 3D glasses, even prescription ones.

The fly in this ointment was the screen. A special screen had to be used otherwise the images came back randomly polarized and the system did not work. Most theatre owners were unwilling to make the investment. The system was replaced with the red/green monochrome system and the public lost interest after the novelty wore off.

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Perhaps you could explain what filter factor means?
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F = 2^S

Where F is the filter factor, S is the attenuation in stops and ^ means "to the power of." Thus if the attenuation is 1 1/3 stops that is the same as 4/3 stops. So the 4th power of 2 is 16 and the cube root of 16 is apporximately 2.5 so the filter factor is 2.5

This seems to be a rather obtuse system but I did not invent it. It is used extensively in the literature so if must be handy to someone.
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I just did a test with a polarizer and the auto-exposure indeed shifted between 1/2 to 1&1/2 f-stops as I rotated the polarizer, however, the filter itself also causes a loss of one f-stop just by putting it on the lens.
</font></font></td></tr></table></td></tr></table></BLOCKQUOTE>This more than likely means that the light was not circularly polarized. The statement that the attenuation of a polarizer is 1 1/3 stops only makes sense if you are talking about circularly polarized light.

I think the attenuation of 1 1/3 stops is a number that mother nature has given us. If it were an agreed upon manufacturer's standard then surely they would have chosen 1 or even two which are much easier to work with. Most filters attenuate either one stop (my 80A) or not at all (my UV filter). These are designed in values.
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I'm relating the angle of the sun as to where it is in the sky versus where you point the polarizer. In some parts of the sky you can get a deeper blue than other parts of the sky. I assume this is due to where the sun is positioned in the sky, however, it could just as well have to do with how blue the sky was to begin with in the various parts of the sky that the polarizer is pointed at. Pointing the camera in wide angle at the sun won't render as blue a sky. (and be careful not to look directly at the sun for more than a brief moment, this can cause eye damage) Through the viewfinder, the blue sky gets more intense as you turn the polarizer depending on the angle/location from the sun.
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I've never heard of such a thing but this result does not surprise me. Since the blue sky is a result of internal reflections in water droplets and reflections generally polarize light a correspondence between polarization of sky light and anglular distance from the sun makes sense.


<font color="#a62a2a" size="1">[ July 24, 2002 03:35 AM: Message edited by: Actor ]</font>

<font color="#a62a2a" size="1">[ July 24, 2002 03:42 AM: Message edited by: Actor ]</font>

[Alex]

Mikey B, it would be real interesting to see how the automatic light meter reacts to a polarizer. Perhaps the needle has more attenuation than 1.5 f-stops because it is so super-sensitive.

Any polarizer that matches your thread ring size should work fine.

Cameraguy, I didn't understand your post.