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I got hold of an ancient film SLR, and have purchased a couple of rolls of film. One of then, a roll of Ilford Delta 400, has an indication about maximum temperature of 24°C. It also has an indication of 27° (which I assume is also a temperature indication.

What does these temperature indications mean? Some of the days I have been shooting have been hotter than 24°C. Will this affect the exposures? Or is this an indication for the development of the film?

Image of Ilford Delta 400 film

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2 Answers 2

up vote 15 down vote accepted

The 24°C (75°F) is the temperature below which the film should be maintained when stored for long periods of time. That is why many film photographers store their bulk film in a refrigerator. It will not harm the film to shoot it in conditions warmer than 24°C, but it will degrade it to store it in higher temperatures for extended periods.

The 27° is a reference to an alternate system of designating film speed known as DIN (named after the organization that defined the standard: Deutsches Institut für Normung). Each 3° in the DIN system equates to a doubling of sensitivity. DIN 30° would equate to ASA 800, while DIN 24° would equate to ASA 200. ASA 400 and DIN 27° are just two different ways of expressing the same sensitivity to light. Although the old logarithmic DIN component is no longer widely used, both it and the old arithmetic ASA component are included in the ISO standard. Digital camera manufacturers seem to stay strictly with the arithmetic scale (Hey, ISO 204800 sounds more impressive compared to ISO 6400 than DIN 54° does compared to DIN 39°). From Wikipedia:

The ISO system defines both an arithmetic and a logarithmic scale. The arithmetic ISO scale corresponds to the arithmetic ASA system, where a doubling of film sensitivity is represented by a doubling of the numerical film speed value. In the logarithmic ISO scale, which corresponds to the DIN scale, adding 3° to the numerical value constitutes a doubling of sensitivity. For example, a film rated ISO 200/24° is twice as sensitive as one rated ISO 100/21°.

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It's widely ignored, but ISO uses both the ASA-type (400) and DIN-type (27°) speed representations. It's not really accurate to say 400 is "the" ISO speed and 27° isn't. en.wikipedia.org/wiki/Film_speed#Current_system:_ISO –  coneslayer Jul 16 '13 at 17:45
    
Yeah, I thought about changing ISO to ASA and then explaining ISO included both once I realized that it did when researching to make sure the equivalents I used were correct. It has been a LONG time since I used DIN for anything! I might edit it yet. –  Michael Clark Jul 16 '13 at 19:25

The second number (27°) isn't a temperature, it is the DIN value -- a different system for measuring film speed. (That's why it's written right next to the ISO rating.)

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