Here is the ISO perspective, reference number ISO 6:1993:
"Black-and-white films will generally provide excellent results in several
different developers and processing conditions. At the same time, it is
realized that the speed of a film depends on the process used. Therefore,
this International Standard specifies a method for determining the photo-
graphic speed of film/process combinations. This means a particular film
may have several I S 0 speeds associated with it depending on the pro-
cesses used. For this reason, it is important that manufacturers indicate
the processing conditions for which ISO speed values are quoted.
This International Standard recognizes that black-and-white films do not
generally have a unique speed if several different processes are rec-
ommended. This conflicts with the tradition of associating a specific speed
value with a particular product. In the future, the process used for deter-
mining speed values should be unequivocally described to avoid misinter-
pretation. Since users often do not know how these films will be
processed, manufacturers have an obligation to provide a speed value for
this situation which will ensure good results. Usually they will take ad-
vantage of the overexposure tonal latitude of the film and give it a con-
servative speed value to protect users from underexposure effects in case
the film is put through a process which yields low speed.
It is recognized that the speed at which a film can be exposed is de-
pendent on the extent of development, scene luminance range, subject
matter, printing paper, etc. This International Standard specifies that
film/process speed is determined when the film is processed to obtain a
specified contrast level. The relative ISO speed ranking of various films in
different process systems will generally differ. The I S 0 speeds will pro-
vide correct exposures for average scenes with exposure meters con-
forming to ISO 2720 or ISO 2721 when the film is processed as specified
in this International Standard.
For the purposes of this International Standard, the
following definitions apply.
3.1 exposure, H: The time integral of illuminance on
the film, measured in lux seconds.
Amounts of exposure are often expressed in logarith-
mic terms (log,&).
3.2 speed: A quantitative measure of the response
of the photographic material to radiant energy for the
specified conditions of exposure, processing and im-
age measurement.
5 Test method
5.1 Principle
Samples are exposed and processed as specified be-
low. Density measurements are obtained from the
resultant images to produce a sensitometric curve
from which values are taken and used to determine
ISO speed."