Black Filter

I went out with a Lita this afternoon to take some pictures of the cherry blossoms. The cherry trees were a gift from the Japanese almost a hundred years ago to us.I took with me the camera, a tripod, some lenses, and a 1.8 neutral density filter. Neutral density filters are measured on a log scale. A 0.3 ND filter, for instance, will block roughly half of the light. Every increase of 1 will drop 10 times the light. So a 1.0 ND lets a tenth light through, a 2.0 would allow a hundredth, and a 3.0 would only allow through one thousandth of the light.So by black filter I'm not saying filter out blacks... that's what the Oscars do. ;-)The filter I had with me was a 1.8 ND -- it blocks six stops of light.I realized that I did not block nearly enough light to take long exposures even stopping down to f/16 with an ISO of 200 (or pulled to 100). Even on the overcast day that we had to work with, I was only able to achieve a roughly two second exposure.I was pretty shocked.Of course I suppose I could have done the math beforehand.So I ordered a 3.0 ND today. Another four stops -- doubling the exposure four more times; 16x.I didn't think I needed that much, or rather that little light. But the math, and reality, is what it is.