# ASA ratings

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### #1 Jack Anderson

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Posted 08 September 2005 - 03:37 PM

I'm a DP and I teach; cinematography at Cal State Long Beach. One of my students asked me about ASA (ISO) numbers. Do the specific numbers relate to anything--do they rfer to any specific scientific formulation--or are they just arbitrarily chosen so that they relate to stops?
Jack Anderson
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### #2 John Pytlak RIP

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Posted 08 September 2005 - 03:54 PM

I'm a DP and I teach; cinematography at Cal State Long Beach. One of my students asked me about ASA (ISO) numbers. Do the specific numbers relate to anything--do they rfer to any specific scientific formulation--or are they just arbitrarily chosen so that they relate to stops?
Jack Anderson

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ISO film speed ratings for still films are specified by ISO standards:

http://www.iso.org/i...ICS2=40&ICS3=20

Motion Picture EI ratings are based on sensitometry, and fine-tuned empirically based on picture tests:

http://www.kodak.com...pdf/tib5209.pdf

http://www.kodak.com...85/cis185.jhtml
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### #3 Dominic Case

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Posted 08 September 2005 - 10:16 PM

Do the specific numbers relate to anything--do they rfer to any specific scientific formulation or are they just arbitrarily chosen

Under the old ASA system , the speed point is defined as that exposure Ex required to produce a density of 0.1 above fog level when the film is developed to an average gradient of 0.62 over an exposure range between logEx and (logEx +1.3). If that speed point is expressed in metre candle seconds, then the ASA speed rating is given by the formula 0.8/Ex.

You might conclude that the specific numbers might as well be arbitrarily chosen

In fact this formula doesn't work for colour motion picture emulsions for a number of reasons (for one thing, the gamma is wrong) - but it's a historical reference point, to the extent that you would probably give the same exposure to a b/w stills negative rated at 100ASA by this method, and a modern motion picture colour negative stock of 100EI.

ASA went out a long time ago anyway, prelaced by ANSI which now defers to (or embraces) ISO.
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### #4 Laurent Andrieux

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Posted 23 September 2005 - 06:38 PM

Under the old ASA system , the speed point is defined as that exposure Ex required to produce a density of 0.1 above fog level when the film is developed to an average gradient of 0.62 over an exposure range between logEx and (logEx +1.3). If that speed point is expressed in metre candle seconds, then the ASA speed rating is given by the formula 0.8/Ex.

I thought it was log(0.8/Ex) ??

Anyway, this is for black and white only...

In color, you don't have one point at given exposure, but one for each layer (R, V,

then, take 1.4142 instead of 0.8 ; and Log Ex = Log (root mean square(Ev +Er)), so that Ev and Er give a density of fog level + 0.15

The formula for reversal is even more difficult to write here...

As Dominic says, only for still photo, doesn't apply to cinema. EI means Exposure Index... you have an Index, it's you to determine your rating, as to use your meter...

But, film manufacturers do things nicely : the EI rating gives a very usauable value as to compute as an ISO value in your meter...

"in the good old days" they would only give you a required illumination for a given T stop.

The formula that "calibrates" photo meters is :

TÂ²/t = I.Siso/245 where T is the required T stop, t the exposure time, I the required illumination (IN LUX,sorry, I'm continetal, not imperial, but you get about 1fc=10.7 lux) and 245 an "arbitrarily chosen" figure
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