Lens Filters: Neutral Density
The main purpose of a neutral density lens filter is to reduce the intensity of light. You might be wondering why would we add a lens filter to reduce the light when we could get the same exposure effect by closing the aperture? Well, the reason you would use a neutral density, or ND, filter is to reduce your light without sacrificing the depth of field.
Why Use a ND Filter?
Besides depth of field, another reason to use a ND filter is to reduce light without sacrificing your shutter speed. The classic example is: what if you are shooting a waterfall? You want the image to look blurred so you use a slow shutter speed. But, slowing your shutter down means you're allowing more light through the lens, and this could over expose the image. To compensate for this, you use an ND filter to reduce the light, get the desired effect, and go home happy about your work.
Selecting the Right Filter
ND filters come in many variations. Here are the most common kinds: ND.3, ND.6, ND.9, ND1.2, ND1.5, ND1.8.
As the number goes up, so does the intensity of the filter. So how do you know which one to use? A general rule of thumb is that as ND goes up in a level like .3 to .6 the reduction of light is about one stop. So the difference between ND.3 and ND1.2 is that 1.2 reduces the light by four stops while .3 only brings it down one stop. Fortunately, in the age of digital photography, you can experiment a little and see the results instantaneously.
All ND filters are grey in color. The darker the color, the heavier the ND. IF you find yourself in a situation where the filters are not properly labeled, hold the filters next to a light and compare their shadows. The ND with a darker shadow is the heavier of the two.
ND also comes in the form of a gel. Gels are cellophane sheets that can be placed over lights to create effects. If you needed to bring a light's intensity down and do not have any other options like a scrim or net, you can use ND to bring it down. You want to avoid that though because gels are fairly expensive.
The practical use for ND in a gel form is for shooting interiors. If you're shooting inside a room with windows and don't want the view outside to be washed out and over exposed, you can put ND on the windows to balance your exposures. To do this, cut the ND to the same size of the window. Next, take a spray bottle full of water and spray the window. Place the gel on the window; the wetness will hold it in place. Next, use a squeegee to flatten the gel and remove the bubbles from the window to make the tint look natural. It's a little time consuming and labor intensive, but the effect looks great.