Linear vs Circular Polarizer

In manual mode, you can use either linear or circular. Auto-focus cameras need the circular polarizers. Some manufacturers offer "ultra" polarizers, which they claim produce a deeper effect; you may or may not want this. It is, after all, the glare that defines the glass or water, etc. and removing too much glare may not give you the look you're after.

There are so many variables affecting polarizers that the results are quite difficult to predict; you must simply test, test, test! As you drive around you are quite likely to change the angle of the light falling on the glass, and you will get different results.

When an effect beyond your control is achieved, the best way of dealing with it is to call it "art." Like the variability of hand-stitching on tailor-made clothing, these "defects" only serve to enhance.

These filters are quite expensive to buy, but relatively cheap to rent; I suggest you rent rather than purchase. You can then experiment and find what works best for you.

Good shooting!
Leo

I think a couple of different things are getting confused. Having a circular filter that depends on a specific degree of rotation for its effect like a grad or a polarizer that screws on to the front of your lense (and is round) is a problem with auto focus cameras only if the front element of the lense moves when this focus shift is taking place.

If the filter is screwed on to an external focus lense and the front of the lense moves, it will move your filter with it. Internal Focus lenses are still round and can take a screw on filter, but the act of focusing doesn't affect the rotation of the front-most element.

I've used 4x4 polarizers and the round screw-on type and I'm not sure the form of the polarizer has much to do with it's quality or effectiveness...

I've used one to shoot through windshields as well and yes, the angle of the windshield is fine. You may want to do some test shots however, because depending on how wide you're going to be, it can be extremely hard to shoot "through" the junk that inevitably collects on a windshield without having it be 'in-focus' enough to cause a problem.

Your question about your angle to the sun affecting the effective position of your polarizer has a short and long answer, but the short answer is 'yes it will'. Ideally, if you are adjusting the polarizer to knock out the reflection of the dashboard in the windshield, you should be able to see what the proper angle is in the viewfinder, but if you are trying to get that more saturated blue sky or other effect, you would ideally use a properly adjusted shoting monitor.

*One sort of 'cheat' trick I use if I'm outside and I don't have the option of a shooting monitor and I'm trying to use the polarizer to get that 'blue sky', is to set the camera to auto iris (pro camera ideally, so you can see the ring on the lense...), then rotate the polarizer until you've determined in what position the polarizer causes the lense to 'open' the widest...you should be pretty close.

Tim Kolb

Hi Tim,
"Circular polarizer" does not refer to it's shape, in which case the filter is commonly called, "round." A circular polarizer is constructed with an extra quarter-wave retarded on the side facing the camera. This is necessary on cameras which use polarization of light for auto-focus.

Most professional video cameras do not require circular polarizers; additionally, the linear polarizers are less costly.

Hope this helps...
Leo

Hi Tim,
I must admit that I find the finer points of the physics of light daunting indeed; here's a quote from Ira Tiffen which may help, "Certain camera optical systems employ internal surfaces that themselves polarize light.  Using a standard (linear) polarizer will cause the light to be further absorbed by the internal optics, depending on the relative orientation.  A Circular Polarizer is a linear one to which has been added,on the side facing the camera, a quarter wave "retarder."  This "corkscrews" the plane of polarization, effectively depolarizing it, eliminating the problem.  The Circular Polarizer otherwise functions in the same manner."

I'm sorry that I can't explain how the cameras use the polarization of light to adjust the focus; perhaps someone else on the list can help us both! While I don't use auto-focus equipment, I have often wished for such a device when doing a moving shot with very long lenses.

Tiffen's entire article can be found at http://www.tiffen.com/camera_filters.htm.
I think it's a must-read.

Good shooting!
Leo

Great discussion of the esoterics of pola filters, but the actual question involves the use in a car of a camera assuming "the driver's point of view". The problem with any pola, circular or otherwise is that the direction of the light (and the accompanying reflections) will change as the vehicles direction does as well! This will require "follow focusing" the rotateing pola in a predetermined way to hopefully accomplish the minimizing of the reflections, which is probably too much to ask.

I think a more certain method of eliminating the reflections would be to construct a solid "topper" or "eyebrow" over the windsheild itself to physically eliminate the reflections from above. Perhaps a roof rack would provide the mounting purchase and a few c-stand arms and a 72x24" solid "blade" would do the trick! Obviously it mwould have to be rigged really solid so it doesn't fly away in motion.

We're fortunate in that each of the video segments from the drivers perspective is relatively short (20-30sec) and we can alter the direction of driving to get the most out of the polarizing filter.

As you say take out all the flares and reflections and we lose what defines the windscreen. I'll take the punt that we can control the flares and reflections with the filter and the way we script and set the shots - the topper isn't really an option for this phase, but something to consider for the next phase.

Thanks all for the advice.

I had a read through some of the Ira Tiffen article last night and have now done so much reading I need to get out there and put some of it into action to cement what I've learned.

However, after contacting Tiffen support about using these Polarising filters with a wide angle adapter it seems that we need to get the filter with a wide angle ring. Here's what they wrote:

"The standard polarizer will cause mechanical vignetting seen as cutoff in the corners. The wide angle polarizer has a cut down, thinner ring which avoids this problem."

Sadly, (although not unexpectedly) the wide angle polarizer costs more.

-Craig.