### Film Temperatures (ISO/DIN confusion)

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There seems to be several different temperature markings on film, and I am curious as to all their meanings. For the sake of example, I am looking at information on Ilford HP5 Plus film. My first question is in reguards to the temperature listed with the film speed, in this case it is shown as ISO 400/27. I assume that the 27 is Celcius. What does this 27 signify? Next I'm curious about EI ratings (found in specifications pdf file obtained from Ilford's website). How are these different from the ISO number and why are there different temperature ratings listed, such as EI 3200/36? Finally I see another temperature listed on the box the film came in along side a picture of a thermometer, that indicates 24C/75F. Is this the temperature to store the film at?

One more that may have been asked before... I know that film stored in a hot environment can fog the film... should I worry about storing the film in too cold of a place? Is a refridgerator that keeps a temperature of 38F too cold for the film?

I appreciate any help on these ongoing mysteries. Thanks.

-- Ryan Sandridge (dieb42@aol.com), June 27, 2000

#### Response to Film Temperatures

I think you are getting confused with the DIN number. The number following the ISO 400/nn is the DIN number which is a number that represents the speed of the film in the same was as the ISO number except that it is a log scale.

ISO 100 = DIN 21 ISO 200 = DIN 24 ISO 400 = DIN 27 ISO 800 = DIN 30 ISO 1600 = DIN 33 ISO 3200 = DIN 36

The DIN numbers increase 3 for every factor of two in ISO.

This number has nothing to do with the "temperature of the film."

They put these numbers on because some light meters are calibrated in the DIN numbers. It makes it easier to compute because adding or subtracting a stop is just simple addition .. the beauty of logrighms.

I am not sure about the too cold question. I suspect that 38F is fine as long as the film has not been opened, and that you take the proper amount of time to let it come to room temperature before opening it.

Somebody else might know wheather or not 38F is too cold or not.

Ken

-- ken Heflinger (khefling@ichips.intel.com), June 27, 2000.

#### Response to Film Temperatures

DIN stands for the norm of the german industry, and Asa is the american norm.

-- Patric (jenspatric@mail.bip.net), June 27, 2000.

#### Response to Film Temperatures

EI (exposure index) is a film speed which has been calculated in a way other than the one specified in the ISO standard. Some manufacturers (such as Ilford) use them because they feel that the ISO standard does not reflect the way in which the film will be developed in the real world. Many photographers also use a personal EI to account for the way in which they expose/process/print film, which often differs from the assumptions of the ISO standard.

With regard to cold, film may be stored at any low temperature desired, so long as it is allowed to warm up to ambient temperature before use (since I live in Alaska, ambient temperature is sometimes - 40 degrees; you have to take film out of the freezer and let it cool down). Many of us store film in the freezer since it lasts much longer.

-- John Lehman (al7jj@yahoo.com), June 28, 2000.

#### Response to Film Temperatures

Until some time ago, there were two major systems for labelling film speeds. One was ASA (American Standards Association), the other was DIN (Deutsches Institut f|r Normung). The ASA system had the advantage of being linear (doubling the number means that the speed is doubled), and the DIN system had the advantage of being logarithmic just like the film (an increase of three degrees DIN also means doubling the speed, or one f-stop more). It has been common for a while to print both scales on cameras, and the ISO (International Organisation for Standardization) drafted an International Standard. The labels 100 ASA and 21 DIN, which are equivalent, are now shown as ISO 100/21.

So there is indeed no temperature involved in this. The temperature shown beside the little thermometer pictogram is probably a warning not to expose the film to too high an ambient temperature for this might cause fogging.

As for the E.I.: There is also an ISO standard for measuring film speed from characteristic curves obtained with standard development. It involves evaluating the density caused by a minimal, just-above-the-threshold exposure and the local slope of the curve. We all know that changing the developer may change the film speed. So the term "standard development" implies that you may encounter deviations with your specific set-up. Many photographers therefore calibrate their films and meters for themselves to exclude this source of error. Such an actual film speed is strictly an E.I., or exposure index.

The second kind of E.I. is that for push, or pull development. Pushing and pulling change the gradient of the curve, the negative contrast. They do not really change the films ISO speed (they do, in fact, but mostly due to their influence on the gradient, and only marginally when compared to their effect on the E.I.). The "speeds" given for push development are then *apparent* speeds, as they are based on metering mid-gray.

So E.I. is actually a fuzzy term which is used for non-standard speed ratings.

-- Thomas Wollstein (thomas_wollstein@web.de), June 28, 2000.