Filter Factors to the rescue: By tradition (and some science) the basic unit of exposure change is the f-stop.

This is a 2X incremental chance in image brilliance. In other words, each f-stop change is a doubling or halving of the light energy that comprise the exposure.

This being true, we can state that if a filter that attenuates 1 f-stop of light it can be described as having a Filter Factor of 2. If a filter attenuates 2 f-stops of light, its Filter Factor is 4.

Another way to compute a Filter Factor, if the number of stops of attenuation is known, we elevate the number 2 using the number of stops of attenuation as an exponent. Using this method a table of Filter Factors can be conducted.

1 f-stop = 2^1 = 2

2 f-stops = 2^2 =4

3 f-stops = 2^3 = 8

4 f-stops = 2^4 = 16

5 f-stops = 2^5 = 32

6 f-stops = 2^6 = 64

7 f-stops = 2^7 = 128

8 f-stops = 2^8 = 256

9 f-stops = 2^9 = 512

10 f-stops = 2^10 = 1024

Now the FF (Filter Factor) can be used as a multiply factor that computes a revised shutter speed.

Suppose your unfiltered exposure is f/8 @ 1 second. You desire an exposure time of 64 seconds. You mount a 6 stop ND and the revised exposure time is the unfiltered time multiplied by the FF. Thus 1 X 64 = 64 seconds.

Another example: Unfiltered exposure time is 1/100 of a second (written in decimal form as 0.01 seconds. You mount a 10 stop ND. The revised exposure time is 0.01 X 1024 = 10.24 seconds.

Reciprocity failure is a photo film phenomenon. We must add time if the exposure is prolonged. Digital sensors have their problems but not this one.