Exposure Calculations

I am puzzled by what Thousand Oaks Calls their Black Polymer Film and Starlite Film to be able to block light upto 16 Stops,that would be ND16, which would be approximately 1/64000 time penetration. Fred Espenak’s exposure guide and Xavier’s guide gives value for ND 5.0 or ND 4.0, should I adjust the values for ND 16, or does the guide mean OD 5.0 or 4.0 which would be close to ND 16 or ND15?

Thank you for your help.

Sanjiv Modi

There are different methods of notation for the density of neutral density filters. The “optical density” is a logarithmic number that is usually prefaced by ND and stated a with one decimal place, as in ND 5.0. To find out how many stops this represents, divide that number by 0.3. So an ND 5.0 filter reduces light by 5.0 / 0.3 = 16.67 stops.

Unfortunately, the notation that is referred to as the “filter factor” also tends to use ND followed by a number, but rarely with a decimal point. The filter factor refers to the increase in shutter time needed to get the same exposure from that filter. The filter factor is the 2 raised to the power of the number of stops. Thus a filter factor of ND 16 reduces light by 2^4 = 16, 4 stops. A 10 stop filter would have a filter factor of 2^10 or 1024, and a 16.67-stop filter would have a filter factor of 100,000.



Thank you Dave, this makes sense.