It just occurred to me that stacking two polarizing filters (not CPL's) should allow me to effectively create a variable ND effect. I haven't tried it yet, and I can't imagine this is an original concept, so I wanted to throw it out here to see if anyone has played with the idea.
What started me thinking about this was a problem that I worked on a long time ago.
Where I used to work we had a long hallway between two buildings. The hallway had BIG windows on each side so of course birds were always trying to fly 'through' them. An often fatal mistake. So we put polarizing film on the windows... vertically polarized on one side of the hall, and horizontally polarized on the other side. From the outside, looking through two windows, it no longer looked like an open space - it was quite dark, but from inside, looking out through a single window, there was only a minor diminution in light. It kept the people happy, and the birds alive.
I've done a fair bit of it Arlen, and within certain limits it works well. The first limit is the rather dark starting point of the pair. The second is the likelihood of color shifts with cheaper filters, and especially if the brands aren't the same. You need to be very concious of light striking the filters, too, as you don't want light rays rattling around between them and resulting flare. A good lens hood helps, but becomes really troublesome when manipulating two polarizing filters. And of course, when stacking filters there's the potential for vignetting. Thin filters are a solution, but only if at least one is front-threaded to allow it.
Edit- One more important point:
Be preprepared for really "flat" light to result in your photos. With NDs you can manage to keep lighting that makes photos so interesting, but using 2 polas can really work against you. I tend to use 2 polas for circumstances requiring fine tuning I can't achieve with NDs, but they're a second choice.
Here's another, more useful approach: Stack a pola with the ND of your choice. I'm betting you will like the results better.
Posted by David Bostedo on October 23, 2008 at 11:09 AM
Hey Arlen - Check out this page on making your own vari-ND (as Singh-ray calls it). It suggests using a linear polarizer as the front filter. I think two circulars stacked can do some weird things.
It also suggests using a really cheap linear polarizer, but that's probably a bad idea image quality-wise. Linear polarizers are cheaper than their circular polarizer equivalents though.
Posted by Steve Sieren on October 23, 2008 at 5:17 PM
Sorry but not a fan of the vari ND by Singh Ray. I get poor effects when used on with my 17-40 @17mm on my 5D. It works if I'm zoomed in a little but I'de rather just use a solid 10 stop or more maybe even stack a couple. Anyone want to buy my vari ND filter 77mm before I put it on ebay? I'll sell it for cheap.
Posted by David Taylor on October 23, 2008 at 11:21 PM
Very good proposal, I've wondered about this for some time as well. Interesting to hear the comments by people that have taken the time to attempt this as well as compare that to other options. Great info here!
Thanks, Arlen, for the thread --and David, for that link. I have a 72-mm linear polarizer from the old days that I've been keeping for I-didn't-know-what (not to use on the 21-mm prehistoric lens I bought it for, anyway).
Now I know what: stick it in front of my 72-mm CPL and see what happens.
I don't recall experiencing that Kent, but in my applications for doubles, I'm almost never using WA lenses. The bigger issues for scenics are dramatic flattening of the light and color shifts.
In scenics with WA's, I'm after slow shutter speeds if I use ND's or pola pairs, and flattening of the light with the latter is a serious problem. The light is usually what makes the shot interesting, and stripping it away turns an appealing scenic into a documentary shot in mediocre light. The ND's allow me to retain the interesting light while still achieving the slow shutter speed.
Posted by David Bostedo on October 27, 2008 at 6:20 PM
Kent, Hank, et al - I've used a CPL and ND together quite a bit, and I've also never seen uneven darkening. Theoretically, no matter what the CPL does to the light, the ND simply reduces the transmission by a prescribed amount. The uneven polarization is because different parts of the scene are at different angles with respect to the light source (the sun usually). It's not that the CPL is darkening part of the scene more, it's just that it cutting more polarized light out of the scene depending on the angle.
It may make some small difference depending on the order (CPL behind ND, versus ND behind CPL) but in normal use, I've never noticed anything unusual.
Discussion
10 repliesWhat started me thinking about this was a problem that I worked on a long time ago.
Where I used to work we had a long hallway between two buildings. The hallway had BIG windows on each side so of course birds were always trying to fly 'through' them. An often fatal mistake. So we put polarizing film on the windows... vertically polarized on one side of the hall, and horizontally polarized on the other side. From the outside, looking through two windows, it no longer looked like an open space - it was quite dark, but from inside, looking out through a single window, there was only a minor diminution in light. It kept the people happy, and the birds alive.
Edit- One more important point:
Be preprepared for really "flat" light to result in your photos. With NDs you can manage to keep lighting that makes photos so interesting, but using 2 polas can really work against you. I tend to use 2 polas for circumstances requiring fine tuning I can't achieve with NDs, but they're a second choice.
Here's another, more useful approach: Stack a pola with the ND of your choice. I'm betting you will like the results better.
It also suggests using a really cheap linear polarizer, but that's probably a bad idea image quality-wise. Linear polarizers are cheaper than their circular polarizer equivalents though.
Now I know what: stick it in front of my 72-mm CPL and see what happens.
Kent in SD
In scenics with WA's, I'm after slow shutter speeds if I use ND's or pola pairs, and flattening of the light with the latter is a serious problem. The light is usually what makes the shot interesting, and stripping it away turns an appealing scenic into a documentary shot in mediocre light. The ND's allow me to retain the interesting light while still achieving the slow shutter speed.
It may make some small difference depending on the order (CPL behind ND, versus ND behind CPL) but in normal use, I've never noticed anything unusual.