### A066844 - Film speeds

A066844 - Film speeds

25, 40, 50, 64, 80, 100, 125, 160, …

In 1876, Ferdinand Hurter and Vero Charles Driffield founded the field of sensitometry when they established mathematical relationships between the density of silver on film, the sensitivity of that film, and the time needed to develop it. Their measurements were rudimentary, but the relationships (now called H-D curves) are still used by modern sensitometrists.

The first widely-used film speed standard was the DIN system, first published in 1931. It was based on base 10 logarithms of the sensitivity of the film, multiplied by ten (similar to the decibel scale). A difference in 3° represented approximately a doubling of sensitivity, and an increase of 20° was 100x more sensitive.

In 1960, the American Standards Association standardized a non-logarithmic film speed scale. In the ASA system, a doubling of the speed was exactly equal to a doubling of sensitivity. Many cameras made during the 1960s and especially the 1970s have both ASA and DIN film speed charts to ease use of film. Finally, in 1987, the DIN and ASA formats were officially merged by the ISO, which supports both an “arithmetic” scale (identical to ASA) and a “logarithmic” scale (identical to DIN). Although use of the logarithmic scale has declined markedly, film must still be officially marked with both — for instance 400/27° or 3200/36°.