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Filters

Tweaking video at the source By Stephen Schleicher
While a great deal of time is spent talking about how to improve an image during post production, very little time is spent talking about way to improve the image at the source - through the camera lens. You could spend some time tweaking the internal settings of the camera, but a better way might be to use a filter.

For those of you who are new to shooting DV, a filter is simply a piece of glass or plastic that is coated to change the properties of light as it enters the lens. Depending on the filter system you are using, you can either have filters that screw directly onto the lens, or you can have filters that fit into a filter holder. The filter holder itself may be a stand-alone unit or part of a matte box system.


The advantage of a filter holder over a screw on filter is that you are able to rotate the filter easily in any direction as well align gradient filters more precisely.

I tend to divide lens filters into four basic categories; protection, neutral density, colored and gradated colored, and special effects filters.

Protection
The most basic type of filter is the UV filter. The UV filter is nothing more than a clear piece of glass which blocks potentially damaging ultraviolet radiation. This filter also cuts down slightly on the some of the blue glare that enters the lens.

UV filters are very cheap and even if they dont stop all UV light, they serve a more important purpose - protecting your more expensive lens. I recommend purchasing a screw on UV filter your camera and leaving it on all the time. This will keep all sorts or gunk (fingerprints, water, dust, flying debris) away from the more valuable coated glass of your lens. If the UV filter gets damaged or scratched, it is less expensive to replace it than your prime lens.

Neutral Density
If you are shooting in an overly bright environment, chances are you are going to have to stop down the aperture to try to obtain the correct exposure. Depending on the situation, you may not be able to close the iris enough to get the desired shot. Enter the Neutral Density filter.

The Neutral Density filter reduces the amount of light entering the camera. The great thing about the Neutral Density (ND) filter is that it reduces a scenes brightness without altering the color in the scene. Depending on the filter company you are purchasing from the ND filter may be called a Gray filter.

Neutral Density filters come in a variety of different grades. Each grade reduces the amount of light coming into the camera. In video, you will typically find ND filters rated at .3, .6, and .9. A .3 ND filter reduces the amount of light entering the camera by 50% or 1 full stop. .6 will reduce the amount of light by 2 full stops and a .9 will reduce the amount of light entering your camera by 3 full stops. That is quite a bit of light reduction. You may encounter situations when 3 full stops are still not enough. In these cases look for higher ND ratings. For example, a ND filter of 3.0 will reduce the amount of light by 10 full stops.

You may ask, "Why not just use the ND filter on my camera?" You can use the built in Neutral Density filter on your camera, just make sure you know how many stops it is taking down your light level. Since many cameras have at most 2 built in ND filters, you are limited in obtaining a specific light level. By adding a ND filter to the front of the lens, better results will be obtained.

Have you ever shot a landscape and when you exposed for the ground, your sky comes out white or too light of blue? Maybe you have tried to shoot a subject against a bright background and when you expose for the subject, the background goes completely white. Want to fix these problems?

If you encounter these issues, you might want to consider a Gradated ND filter. This is a filter that starts out at the specific ND setting on one side of the filter and then gradually becomes clear.

A shot with no filter.


With this type of filter in place, you can line the transition area of the filter with the horizon of the shot. Light will pass through the clear section of the filter normally and the ND portion of the filter at a reduced level. This has the effect of deepening the blues of the sky in a landscape shot. By rotating the filter on its side, you can underexpose one side of the frame. This will allow you to place your subject in the clear area with the proper exposure and have the background visible.

A Gradated ND filter causes the sky to stand out more in the shot.



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