The speed point is a point on the curve from which the parameter called speed is measured. Speed is the term for the sensitivity of a film emulsion to light, sometimes called General Sensitivity.
The usual method of calculating a speed value is to define a speed point as
being a particular point on the curve defined by it's density and to read off
the Log E value it corresponds to.
This will give a value that increases in numerical value the higher the Log E
needed. Since to our senses "speed" implies a higher value the less
the exposure needed to achieve a specific density, the number is usually
subtracted from a fixed value to give such a scale, and then multiplied by some
convenient number to avoid decimal points! Such is the relationship between
speed and photographic speed!
For example, the Speed Rating of Eastman Colour Negative Film, as used throughout the world is:
ER = 100 [1- Log E s]
where Log Es is the Log E value corresponding to a point on the Characteristic curve 0.10 above D.Min.
There will be three ER values, one for each curve, R, G, and B. In practice this formula will result in a number somewhere about 250. This absolute value is thus a combination of a precise Log E, at a precise position corresponding to a low shadow exposure, expressed in a formula that is simply a convenient convention.
Speed values used by the cameraman to determine the correct camera exposure are often quite different to those used in laboratories.
For example, two of the current methods used for camera work are called after the standards institutes that devised them. The ASA, the American Standards Association, rating of a film is based on the amount of exposure required to produce a density of 0.1 above the fog level [in exactly the same way as ER above].
Development has to be in a specified formula developer at specified conditions, for monochrome and at the manufacturers process conditions for colour. The calculation used is different to that of ER. The I.S.O. (International Standards Organisation) value is identical in principle and value to the ASA.
D.I.N. (Deutsches Industrie Normaliche) is another similar system but whereas the ASA and ISO are arithmetic series, the DIN is a logarithmic speed series.
In arithmetic series the speed value doubles with a doubling of the actual speed. In a logarithmic speed the speed value increases by 3 [Log 2] when the photographic speed doubles.
In archive restoration work little value is placed on absolute speed and only relative differences are useful. A technician needs to know that batch A is two printer points faster than Batch B, or that a process has altered in speed by one printer point.
The exact speed in I.S.O./A.S.A. is of no interest. The manufacturers quote speed ratings to laboratories in terms that are unique to the stock.
Camera materials are given Speed Ratings, which are absolute as indicated above, but printing materials are usually given relative values. Kodak uses, for example, the term Relative Printer Rating [RPR].