hello! here's an easy one (hopefully): when stacking neutral density filters, do you add or multiply the filter factors together? for example, if i stack a 10-stop ND filter and a 3-stop ND filter, is the total filter factor 13 stops, or 30?

(yeah, i know, i'm a nut-- this means REALLY long exposures even in sunlight...)

~chris jordan, Seattle

-- chris jordan (www.chrisjordanphoto.com) (cjordan@yarmuth.com), June 22, 2001

Answers

You add them. If the original speed was 1/500s, adding the 10 stop filter will require a speed of 2s (10 stops lower than 1/500s), and adding the 3 stop filter will lower the speed to 16s (3 stops lower than 2s and 13 stops lower than 1/500s) to which you will have to compensate for reciprocity failure.

-- Georges Pelpel (gpelpel@home.com), June 22, 2001.

You have to multiply the factors the one as 8X (3 stops), 64X (6 stops) the stops is the exponent for number two ( 2^3=8, 2^6=64)

The number of stops and the density must be added, the density is the log of the factor

Eg: B+W 110 ND has factor 1000, stops=10, density=3.0 Eg: B+W 103 ND has factor 8, stops=3, density=0.9

Really 2^10 is 1024 but they say just 1000.

Hope it helps (sorry for my english)

-- Jorge Garcia (garjor@hotmail.com), November 23, 2001.