Polarizer / ND question

[Greg D]

I've read discussions here and elsewhere about the use of stacked polarizers (linear on top of circular) for a variable ND filter.  After thinking about the various pros and cons I thought I'd give it a try.  Then a possible "con" I hadn't seen mentioned occurred to me.  Since the degree of filtering of a polarizer is dependent on the angle of light, wouldn't you get very little filtering (only the 1-1.5 stops each that the unpolarized position provides) if the light was behind you (or directly in front, or diffuse as at dawn or dusk)?  If this is correct, it would mean that in such situations you'd have a total of only 3 stops or so.  Is this right or am I missing something?
Thanks

[PeterAit]

I've read discussions here and elsewhere about the use of stacked polarizers (linear on top of circular) for a variable ND filter.  After thinking about the various pros and cons I thought I'd give it a try.  Then a possible "con" I hadn't seen mentioned occurred to me.  Since the degree of filtering of a polarizer is dependent on the angle of light, wouldn't you get very little filtering (only the 1-1.5 stops each that the unpolarized position provides) if the light was behind you (or directly in front, or diffuse as at dawn or dusk)?  If this is correct, it would mean that in such situations you'd have a total of only 3 stops or so.  Is this right or am I missing something?
Thanks

I have not tried this technique, but if I understand correctly, the answer is no. Even if the scene itself contains little or no polarized light, the first polarizer removes most of the light that is not polarized at a certain angle. Thus, the 2nd polarizer has highly polarized light to deal with and can cut out more or less of it depending on its angle.

[Panopeeper]

Since the degree of filtering of a polarizer is dependent on the angle of light, wouldn't you get very little filtering (only the 1-1.5 stops each that the unpolarized position provides) if the light was behind you (or directly in front, or diffuse as at dawn or dusk)?

If one filter holds back half of the light and the other one holds back the other half, then you make a black frame. On the other hand, if you adjust them in the same direction, then the second one does not filter anything. The two filters act independently, together they can filter all light or only as much as one filter would.

Note: it is important, that the top filter is linear; that way the two filters act together.

[EricV]

A circular polarizing filter is simply a linear polarizing filter plus a quarter-wave plate.  The linear filter does all the photographic work (darkening skies, cutting through reflections).  The quarter-wave plate is there only to convert the linearly polarized light transmitted by the first filter into circularly polarized light, which can then pass through the beamsplitter and AF system of a modern camera.  

If you stack a linear polarizer on top of the circular polarizer, then the new linear polarizer does all the photographic work, the second linear polarizer blocks a variable fraction of the light passing through the first polarizer, and the quarter-wave plate again produces circularly polarized light.  If the two linear polarizers are parallel, the second one does nothing; if the two linear polarizers are crossed, the second one blocks nearly all of the light transmitted by the first one.  

If you have followed all of this, here is a quiz: why would it not work to stack the circular polarizer on top of the linear polarizer?

[wolfnowl]

If you have followed all of this, here is a quiz: why would it not work to stack the circular polarizer on top of the linear polarizer?

The quarter-wave plate is there only to convert the linearly polarized light transmitted by the first filter into circularly polarized light, which can then pass through the beamsplitter and AF system of a modern camera.

Good enough answer?  Too vague?

Mike.

[RodK]

I've read discussions here and elsewhere about the use of stacked polarizers (linear on top of circular) for a variable ND filter.  After thinking about the various pros and cons I thought I'd give it a try.  Then a possible "con" I hadn't seen mentioned occurred to me.  Since the degree of filtering of a polarizer is dependent on the angle of light, wouldn't you get very little filtering (only the 1-1.5 stops each that the unpolarized position provides) if the light was behind you (or directly in front, or diffuse as at dawn or dusk)?  If this is correct, it would mean that in such situations you'd have a total of only 3 stops or so.  Is this right or am I missing something?
Thanks

First most of these answers have insight, but miss this.  The ND factor of a polarizer is always the same no matter if polarizing or not.  Approximately 1 2/3 stops.  so 2 polarizers would 3 1/3 stops.   2 polarizers can however block light and may work
for more than that as one writer surmised above,  However with 2 I question the AF and or, meters abillity to cope with the combo correctly.
Rod