When photography was developed into the Dry Plate era, films were produced by specialised suppliers, therefore a standard unit of sensitiveness was needed in ordering to let users expose properly. There were several systems, though all were based on the Characteristic Curve created by Mr. Ferdinand Hurter and Mr. Verio Charles Driffield, but the formula each used was different from one another. Finally, two systems remained, they are DIN (Deutshe Industrie Norm) and ASA (American Standards Association).

The initial point on the Characteristic Curve is called 'basic fog', adding 0.1 density gives the 'Speed Point'. DIN is based on 10 X log₁₀ (rated at 1/exposure value), therefore it is by a unit of 3 for each grade, e.g. DIN 15, 18, 21, 24 etc.

ASA is based on the corresponding exposure value against the density of Speed Point plus 0.80 (rated at 1/Exposure Value), therefore it is by a multiple of 2 for each grade, e.g. ASA 25, 50, 100, 200 etc.

In theory, DIN and ASA are comparable, DIN 18 is equivalent to ASA 50, DIN 21 is equivalent to ASA 100, but there is vital difference.

For manufacturing films and scientific imaging, basic fog is an important value, which is the basic of DIN. For general use, the contrast is more important to give a visible image, which is the basic of ASA. That is why both systems existed for so long. Before ISO was introduced, Kodak, the major supplier of Professional films, had specialists testing each batch of such films and printed in red color on the instruction sheet inside each package, the actual ASA. According to our finding, ASA 100 film, might be about 70% with a tested result of ASA 80, about 25% with a tested result of ASA 95 or ASA 105, about 5% with the exact ASA 100.