The Singh-Ray 77mm Vari-ND Variable Neutral Density Filter - This neutral density filter gives you the capability to use fast film in bright sunlight. ND filters have several uses and offer the possibility to achieve otherwise unachievable results. They enable the shooter to adjust exposure without affecting color balance. ND filters reduce the amount of light reaching the film.
When used with a digital camera, you can effectively see how this filter works. After the initial exposure, you can check your LCD image and then make whatever change in exposure you feel you need to enhance your image. By rotating the ring, you can either increase or decrease the ND effect. This will be especially useful when trying to achieve "blurred" or "panned" shots.
The filter contains "index" points to help you mark your spot. They do not represent actual ND settings , i.e. non-calibrated - but are there to help you re-set your filter when you wish to recreate the effect.