[APML] Physical equivalent of ISO film sensitivity

[Emmanuele Sordini <vega@ulisse.it>]

Dear All,
here are some questions a bit more on the technical side than usual.

Does anybody know if there is a physical equivalent to ISO film 
sensitivity, i.e. how much energy flux must strike a unit area of the film 
of known rated sensitivity at a given focal ratio, for a given exposure 
time, to get a correct exposure?
Of course somebody might argue what it actually means to expose a film 
correctly, perhaps in terms of film density: that's the other question.

The rationale behind my questions is to better understand the criteria 
according to which many astrophoto exposure time tables are compiled. This 
is mainly true of objects with high specific brightness (i.e. per unit 
area); for faint and large bodies like nebulae of galaxies, times increase 
drastically and therefore reciprocity failure must be factored in.

Any insight of any kind will be greatly appreciated.

Emmanuele Sordini


--  APML Archives at <http://www.system.missouri.edu/apml/>  ---
                Unsubscribe at <majordomo@seds.org>