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,”

Back to Basics: Film Speed · Lomography

Understanding Reciprocal Exposures and the Exposure Triangle » Shoot It With Film

When to Use Different ISO Film Speeds – Guide to Film Photography

Film Speed - Adelaide Film Photography Tips

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

Understanding How ISO Affects Images, by Vincent T., High-Definition Pro

Intro to ISO and Image Grain/Noise for Photographers (Shooting in Manual Part 5) - Hunter and Sarah Photography

Minimum Shutter Speeds for Handheld Shooting: The definitive answer to How slow can you go - Improve Photography

ISO - ISO 6 — Camera film speed

Film Speed and its importance

Beginner Guide - Basic understanding of ISO, Shutter Speed and Aperture - Tahusa