BTW, while waiting from my Cavision 3" 0.6 ND glass filter, I tried a cheap Fotga resin one this morning. Everything's magenta and blurry.

@Sangye

I didn't really assume you were going to use a round filter.

I certainly didn't expect you to use a cheap filter.

We are al' of us, looking for good, affordable glass filters.

A polarising filter is not a real ND filter. If it does ND, it's more the way a dancing bear dances. (Not that I have anything against bears - it's just that Smokey is no Fred Astaire).

Use polarisers and that's another whole look. You want richer colour? Polarise. [Polarisers reveal more colour by eliminating the reflection sitting on top of the colour].

Motion picture film, unlike still photography, loves the same reflections which would confuse the viewer's eye when looking at a still pic. Give us moving glass doors, water, eye-lit portraits - movies thrive on it.

Polarise, so as to darken? Well, that's a rare set of requirements. It probably comes up from time to time. Can't remember when.

Yes, as you say - and as I am saying, a good set of fixed ND filters is what a matte box does best. I have not yet seen a film set with a rotating ND filter except graduated ones.

You can even get away with a .6 ND most of the time where I work. In cloudier places a .3 will complete the kit. (After all, we have other settings to play with like ISO and shutter).

You have two additional glass-air surfaces. I am not even talking that even single polarizers have usually worse image quality compared to good ND filter.

Why is it, that film sets use ND kits rather than variable NDs?

Dunno. But I remember having to write the ND on a slate. We were crazy about record-keeping.We would never have been able to measure a true ND value if it were variable.

@Sangye

Variable ND filters are just two polarizing filters attached to each other in such a way that they can rotate. When you combine two polarizers and rotate one of them, you have created a variable ND filter.

You have explained it perfectly.

why not do this using the filter trays in a matte box?

Because you're inevitably polarising at the same time, so you usually lose your reflections.

swapping out ND filters all the time

Takes half a second. Use that belt-pouch they come in. Practise.

Plus, the matte box looks seriously cool with that expanse of glass in front. You look even cooler swapping filters. :-)

@Sangye

If this online DSLR light calculator seems to be of any help, please tell me how you did it:

http://www.hdslr-cinema.com/tools/light.php?crop=2.0&cust_crop=&aper=0&iso=160&fps=24.000&cust_fps=&nd=0&stop=0

And again, f/0.95 in daylight? Why?

As I read this, I was watching a series of local TV ads, a couple not so good but with wide lens/aperture shallow DOF combinations - yet, the talent in the ads were in front of sports grounds and open countryside.

more like, "Shallow DOF? Why?" Just why do you want to isolate a footballer from the football ground? Is it so show his feelings of solitude? Is the POV from binoculars? In whose time is the shot?

In these ads, it was clear that the videographer had no idea or any purpose.

A much -desired shallow DOF look has suddenly become uncool for me. Overdone, soo 2012. Already, as of 4th October 2012 I'm over the gratuitous, inappropriate shallow DOF.

@Sangye Here's the thing (and this applies to the 4x4 linear and circular polarizer approach as well): You don't necessarily want to polarize every shot, which is what you will be doing to some degree by always having a polarizer in front of your lens. If you rotate a fader filter in the screw threads on the front of your lens while shooting a reflective surface, you'll notice the shift -- and if you screw the filter on tight to keep it from falling off your lens, you really have no control on the orientation and thus polarizing properties of the rear filter element.

As @goanna says, this defeats the "neutral" purpose of neutral density. Sure, it's great to bring out the sky on landscape and wide shots but there is a reason why DPs tend not to polarize their human subjects. It literally cuts the subtle skin reflection that most of us don't even notice until we get to post. Then skin looks very flat. I offer the attached examples. They are a bit extreme but you should get the idea.

Also, I'm not concerned about the polarizing effects of stacking polarizing filters. VNDs are built this way, and they do not polarize an image in the same way that a single polarizing filter does, nor in a way that I have ever found problematic, bokeh striations notwithstanding. From what I understand, when two circular polarizing filters are used together with one in reverse, the second polarizer effectively "de-polarizes" the image. This is consistent with what I've read elsewhere, and explains why brands like Singh-Ray sell polarizing as well as non-polarizing VNDs. One uses two circular polarizers, while the other uses one linear and one circular polarizer.

The only thing that has been posted so far that gives me pause, is VKs point about minimizing air-glass surfaces, and that effects aside, polarizing glass negatively impacts IQ more than neutral density glass. I'll probably borrow some circular polarizing as well as ND filters from a friend and experiment with this, and then make a decision.

@Sangye: If somebody wants super shallow DoF I can see that, but what I do then is to slap on a longer lens. Naturally shallow and it adds a lot of other nice effects. Plus in exteriors distance isn't an issue...

But I would recommend getting a regular ND (I use a 1.2, as the GH2 is very sensitive to highlights) in 4x4 and then a Vari nd as a screw on. And it has A LOT of polarising effect. We shot with one on an Epic and as soon as it was on, I saw the sky drop, not from the ND, but from the pola...

@QuickHitRecord

Yep, that's what we're saying! Thanks.

Those reflections in water are essential in that shot.

The earlier still shots of hands? The importance of reflectivity is less obvious under certain circumstances. But the principles of studio lighting demand reflection off eyes, hair and clothes' sheen (for back lighting).

FWIW, they say Louis Armstrong never really had a perspiration problem. They sprayed him with water before each take to bring out his skin texture. He played along.

Lighting/filming dark faces is a whole other rule-book. It's what I do most and it still poses problems. These days, I am extra respectful of people's sensitivity when I mention skin colour.

With polarisation testing: It's so easy to do ,why not do the test anyway instead of googling? (Unless you're generation-Z, in which case put in on your bucket-list of real life experiences) ;-)