Exposure ¹⁾

Table 1 contains a complete list of related exposure data. Column 1 gives the percentage of light transmission, column 2 the appropriate exposure factor. The absorption of the filter can also be compensated for by means of increasing the aperture ( column 3 ). This column clearly shows that most cameras are not able to adust adequately to low densities. 30% absorption, equivalent 70% transmission, only means half a lens stop!
 

A common term in connection with gray filters is neutral density ( column 4 ). The list has the advantage that gray values may be calculated by addition. If one compares whole-number aperture values with their neutral densities, it will be seen that the value ND 0.3 corresponds to one lens-stop, 3 x 0.3 = ND 0.9 three lens-stops and so on.

The light value to be found in column 5 is a measurement dependent on the aperture but also on time or the antiquated DIN value.

All data in columns 2 - 7 are based on the principle of doubling or halving light quantity. This is easily seen by comparing each column. Only the doubling value is different.

All other values, too, follow the principle of halving or doubling, if not always in such a clear-cut manner. Thus, 3 DIN correspond to 1 stop and in the case of ASA values, film sensitivity data is adjusted to a continuous doubling of light sensitivity.