ND Filters vs. Polarizing Filters: When to Use Each

[SALLYNABIL]

I just wanna know the difference between neutral density filters and polarisation filters and when is each of them used?

I was watching Atonement the other night and noticed that the light was very bright in the morning scenes, whether interior or exterior, to the extent that I felt that some scenes are overexposed. Does this result from dispensing with any of the previously mentioned filters, for example?

 

[Red Prince]

A neutral density (ND) filter is simply a gray filter. It reduces the amount of the light passing through, and it does so uniformly, so it has a similar effect as loading a slower film into your camera.

A polarizing filter only allows unpolarized light to get through. One of the ways of light to get polarized (if I recall my high school physics correctly) is by it being reflected. So, for example, if you are looking at a car and the light of the Sun is being reflected straight into your eyes (or your camera), the reflection is blinding. But it is polarized, so it would be filtered out by a polarization filter. So, the main advantage of a polarization filter is to reduce glare.

To be more technical, a polarizing filter only lets through the rays of light moving in one direction and blocks out everything else. If you turn the polarizing filter, the direction changes. That makes it possible for you to decide which light gets through and which is filtered out. That also means that you may need to turn the filter to reduce the reflected glare mentioned above. And that lets you control how much or how little of the glare to let through.

These pages explain polarizing filters (you need to click on next at the bottom of each page to read the whole story).

 

[SALLYNABIL]

Thank you for your useful participation.

I just wanna go back to Atonement. I felt that the lighting in some scenes was v.bright as if overexposed. I liked it but just wanna know how they can do this.

 

[Red Prince]

Sorry, I have not seen Atonement, but I am sure someone else will answer that question.

 

[David Mullen ASC]

"Atonement" was lit and exposed for that look. ND filters just reduce overall exposure, they don't make hot areas hotter in relation to dark areas. As for Polas, if there is no reason to use them, then you don't use them. I didn't see many shots in "Atonement" that would have required a Pola filter, except for some of the silhouette shots against the ocean -- perhaps some glare was reduced with a Pola but perhaps not.

But they probably used ND's because they had nets on the back of the lens, so in order for the net pattern to not come into focus, they had to shoot at a nearly wide-open lens aperture -- so if the light outside was reading an f/5.6 on the meter, he would have used an ND.9, let's say, so he could shoot at f/2 instead. But the hot spots indoors were due to using bright lights and exposing them so they read as hot, and when outside, by shooting when possible, in strong backlight and exposing more for the shade.

Also, the movie was digitally color-corrected, so it would have been possible to increase the contrast in post and cause bright areas to burn out even faster.