Depending on your experience in photography and film, you may or may not have come across different filters. Some types of filters can be screwed onto the front of your favourite 35mm SLR lens, while others can be used along with a Matte Box. Either way, having a good working knowledge of various filters can be a great asset as a photographer or DoP/ Camera Operator.
I took two filters out to the Philippines with me: A Tiffen 77mm Variable Neutral Density (ND) filter and a B+W 77mm Circular Polarising Filter (CPL).
I primarily used the CPL. Being in a tropical location with lots of bright skies, ocean and sand all around me, I needed a way to be able to capture the amazing scenery and still expose people correctly.
With an ND filter, the light is cut evenly across the input (RGB) channels. This is a great way to reduce the overall amount of light hitting the sensor, without changing the aperture, shutter speed or ISO (sensitivity) settings. However, if there is still a strong contrast between the brightness of the sky and the subject on the ground, the problem has yet to be solved.
One way to get around this is to use a Gradient ND filter. This will often have a heavier ND grade on the top portion of the filter, in order to lower the brightness level of the sky. This would allow the camera operator to expose both the sky and subject correctly. This can be great tools, but may not always be the right tool for the job.
A second way to get around this problem is to use other lights or reflectors to brighten the subject on the ground. This would be the opposite approach to using a gradient ND filter. Instead of lowering the brightness of the sky, you increase the brightness of the subject in order to expose both the subject and the sky properly.
HOWEVER, it is not always possible to run around with a reflector and ask the subject to repeat whatever action you wanted to. This is especially true when shooting more of a documentary style, or a run-and-gun style. More often than not you only get one chance to capture what you need. If this is the case, or you don’t want to bother with extra people holding reflectors or adjusting your framing to match the gradient ND filter, a CPL may do the trick.
Circular Polarising Filter
Without getting into the technical details that I wouldn’t be able to properly explain anyway, a CPL works by basically blocking light based on different orientations of the reflected light. This can be used to see through reflections in windows, see through water a bit more clearly, or add a vibrancy to the sky and clouds. By turning the CPL in front of your lens, it quickly becomes apparent at which orientation the light is blocked out.
Here is a quick example of my CPL blocking out the reflections from the surface of the ocean:
A CPL can also allow you to expose the sky and ground evenly, without having to use additional filters or a sometimes awkward gradient ND filter. By blocking some of the light form the sky, it effectively reduces the brightness in the sky. The example below is a scene that would have been very difficult to capture with a gradient ND filter, or no filter at all. The horizon is flat, but the group of trees gets in the way of the horizon as well.
The video shows a comparison between the ungraded clip and the slightly colour graded clip. *I didn’t think ahead to get a shot without the filter… sorry!
There are plenty of other images and videos of a CPL in action. Just do a quick google search!