What is film speed, and which ISO should I use when shooting?

If you’re used to the digital world, film speed may not be a term you’re immediately familiar with. However, you may have heard of “ISO,” since it’s made the leap to modern digital cameras. The simple equation: ISO is film speed. 200, 400, even 800 — those numbers listed on your film’s packaging is the ISO and it tells you how sensitive your film is to light.  ISO is the International Organization for Standardization which oversees, develops and publishes technical, industrial and commercial standards worldwide. Historically, various metrics were used to determine film sensitivity around the world - the most common were DIN (Deutsches Institut für Normung) in Europe and ASA (American Standards Association) in the United States. In 1974, ASA and DIN were combined to provide the current ISO system; typically, this is listed on the packaging as ASA/DIN.
If you’re used to the digital world, film speed may not be a term you’re immediately familiar with. However, you may have heard of “ISO,”

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