Neutral Density Filters are used as a means to control exposure.
The denser the filter, the more you must compensate by opening up the lens iris.
Neutral Density filters are built in one stop increments, (see above photo.) The densest filter I have seen and used is a ND 2.10 - equaling 7 stops of attenuation. This filter would be used outside in very bright conditions; snow, beach or desert in the middle of the day to reduce the f stop to exist within the iris range of a lens. A 7 stop compensation from f 45 is f 4.
I have used this filter only a couple of times and that was to achieve a very shallow depth of field to support a story point. Ordinarily, 4, 5 or 6 stop ND filters are sufficient for daylight conditions.
As daylight dwindles, heavy ND filters are exchanged for lighter ones until at some point late in the afternoon, no ND filter is required at all.
Application - For Day Exterior Photography, I generally like to shoot around f 5.6. In those bright conditions, I’ll add the appropriate ND filter that will get the lens in that 5.6 range. For instance, if a light meter indicates that for proper exposure the iris in the lens should be set to an f16, I’ll add an ND 9 to necessitate opening up 3 stops. Three stops open from f16 is a f5.6.
With the same meter reading, (f16) an ND 1.2 equals 4 stops so the adjusted f stop would then be an f4.
To achieve more depth of field, the lens has to have a smaller iris opening: f8,
f11 or f16. Meaning that little or no ND would be added to the front of the lens.
As a Cinematographer, it is completely up to your discretion what ND filter you choose – if any at all.
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