The degree of light blocking is often referred to in terms of the number of ‘stops of light’ which the filter blocks. For each ‘stop’ of light blocked, the exposure time doubles. An alternate terminology uses exponential values of the number ‘2’ to reflect this principle. Yet another system uses values of 0.3 to designate 1 stop of light blocking.
For instance, an ND filter which blocks 2 stops of light can be called a ‘2 stop’ ND filter. Using the alternative terminology it can also be called an ND4 filter (2^2 =4) or an ND 0.6 filter (0.3X2 =0.6). This terminology also applies to the strength of GND (graduated neutral density) filters.
| ND | OPTICAL DENSITY | F-STOP REDUCTION | TRANSMITTANCE % |
| ND2 | 0.3 | 1 STOPS | 50% |
| ND4 | 0.6 | 2 STOPS | 25% |
| ND8 | 0.9 | 3 STOPS | 12.5% |
| ND16 | 1.2 | 4 STOPS | 6.25% |
| ND32 | 1.5 | 5 STOPS | 3.12% |
| ND64 | 1.8 | 6 STOPS | 1.56% |
| ND128 | 2.1 | 7 STOPS | 0.78% |
| ND256 | 2.4 | 8 STOPS | 0.39% |
| ND500 | 2.7 | 9 STOPS | 0.2% |
| ND1000 | 3.0 | 10 STOPS | 0.1% |
| ND2000 | 3.3 | 11 STOPS | 0.05% |
| ND32000 | 4.5 | 15 STOPS | 0.003% |
| ND100K | 6.0 | 20 STOPS | 0.0001% |
