In a previous discussion, I talked about Optical Density Numbers. In this discussion, I will speak about Filter Factor Numbers. Just like in the case of Optical Density Number, the Filter Factor Number is often prefixed by the letters ND. So, an ND 0 means a clear filter without any light-stopping power. Then comes an ND 2, which has a light-stopping power of 1. That means the filter stops half the light that would typically enter a camera.
Next is an ND 4 filter with a light stopping power of 2 stops. That means it allows only 1/4th of the light that would typically enter the camera.
The concept is similar to Optical Density Number, except the naming convention differs.
How do you compensate for an ND filter?
When you use an ND filter, you must calculate on the fly how much light it’s stopping. Let’s say; in the above situation, an ND 4 filter stops two stops of light. That means it allows only 25% of light through compared to a normal position without a filter.
You can push the exposure by two stops to compensate for the light loss. Of course, there are creative possibilities for that. Let’s say that you’re shooting in broad daylight. You have an f/1.8 portrait lens and want to take a picture using the widest aperture (f/1.8).
Generally, if you attempt that shot under bright conditions, you will burn the exposure. Blown-out highlights, skin tones burned, and anything that might go wrong will go wrong. The solution is using an ND filter to ensure less light enters the scene.
With an ND 4 filter, you can reduce the shutter speed to compensate for the light stopped from entering the camera and yet be able to take a well-exposed photograph. Let’s say that the normal exposure at f/1.8 was 1/1000 sec. You use an ND filter with a Filter Factor Number of 16, which stops four stops of light. That allows you to use four stops slower shutter speed without tinkering with the aperture number.
So, you get to keep the nice background blur that your f/1.8 lens can produce. At the same time, because you use the ND filter, the exposure isn’t blown away.
There are many other situations in which an ND filter can be useful. You can use it to make long exposures of moving subjects, like clouds, water, and sea beaches, blurring the effects of clouds and water to create a dreamy effect.