Mechanism
For a ND filter with optical density d the amount of optical power transmitted through the filter, which can be calculated from the logarithm of the ratio of the measurable intensity (I) after the filter to the incident intensity (I0), shown as the following:
- Fractional Transmittance (I⁄I0) = 10-d, or
For example, on a very bright day, one might wish to photograph a waterfall at a slow shutter speed to create a deliberate motion blur effect. In order to do this, one would need a shutter speed on the order of tens of seconds. There might be so much light that even at minimum film speed and a minimum aperture such as f/32, the desired shutter speed would still let in too much light and the photo would be overexposed—or, if shooting at aperture priority, the camera would select a shutter speed that would be too fast to achieve the desired effect. In this situation, applying an appropriate neutral density filter is the equivalent of stopping down one or more additional stops, allowing for the slower shutter speed and the desired motion-blur effect.
Read more about this topic: Neutral Density Filter
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