Figuring ASA Numbers

[Mike Panczenko]

I have been wondering how they find out ASA numbers for quite some time. The only information I can find refers to me a rather expensive ISO publication (5800:1987). I was wondering if anyone (perhaps Mr. Pytlak) can explain to me in the processes that are gone through to get an ASA/ISO number. Thanks.

[John Pytlak RIP]

From the Kodak website:

 

http://www.kodak.com/US/en/motion/support/faq/exposure.shtml

 

Why is the speed rating of motion picture camera films given in Exposure Indices rather than ASA or DIN values?

There is no ANSI standard to determine the speed of these films.

 

The speed of motion picture camera films and the suggested filtrations are determined on the basis of practical picture tests. Suitable safety factors have been included to allow for differences in cameras, variation in lighting, etc. The exposure index values should not be regarded as numbers which express the absolute speed or sensitivity of the film, neither should they be regarded as fixed values which can not be changed if the results of repeated tests indicate the need for such changes.

 

Other information:

 

http://www.kodak.com/US/en/motion/support/h1/exposure.shtml

 

http://www.kodak.com/US/en/motion/support/h1/structure.shtml

[Laurent Andrieux]

It is a fact, however, that if you type in your meter the EI number, you'have a good basis for this trials. If ASA/ISO is not applyable to moyion picture, the EI number is, I guess, as close as possible to what should be the sensitivity to type in the meter.

 

I'm not sure it answers mapanc1's question, anyway, the question could be "how is calculated the ASA number, say for still pictures". I guess there is a page somewhere on kodak's site to answer tehe question, Mr Pytlak ?

 

I own a Kodak's book that explains it. It's different for negative and reversal and for printing papers. There are different ways of calculating it from the typical HD curve of the film, I remember there are Kodak, Agfa and OAC method...

 

But... maybe what you might need is a relationship beetwen F stop, required light, speed and sensitivity ?

 

I know a formula : N²/t = (E.Siso)/X where N is the T stop, t the exposure time, E the incident light in lux (lumination) and X a factor, wich varies according to different sources beetween 245 to 270.

 

Would ypou confirm that, Mr Pytlak ?

[John Pytlak RIP]

The calculations of speed for still film, based on sensitometric curves, are fairly detailed, and covered by ISO and ANSI standards:

 

Exposure index:

 

http://www.kodak.com/US/en/motion/support/faq/exposure.shtml

 

Calculation of ISO film speed for still film color negatives:

 

http://webstore.ansi.org/ansidocstore/prod...ISO+5800%3A1987

 

B&W still film speed:

 

http://webstore.ansi.org/ansidocstore/prod...ku=ISO+6%3A1993

 

Color reversal still films:

 

http://webstore.ansi.org/ansidocstore/prod...IT2%2E21%2D1994