A film's ISO, in practice, is the number, referred to as the speed, assigned to the light sensitivity of the film. Nominal ISO is the speed photographers found to be the best for each film and is often not the assigned ISO.
Here’s an example. Portra 800 is a technically an 800-speed film but you don’t get the best results treating it as 800-speed. Photographers found that the best results for Portra 800 are achieved by shooting it like it’s a 400-speed. That makes 400 the nominal speed of Portra 800.
You’re probably wondering why they don’t just make the assigned film speed and the nominal speed the same. To answer that question, we have to go back in time to the 70s when photography was quickly becoming the hobby of every dad in the neighborhood. With the invention of quick-develop labs and more affordable cameras, film manufacturers were facing fierce competition to make faster and faster speed films to entice a new customer base who were focused on the practicality and price of the film, not necessarily the look of a specific brand.
Because of this film “arms race,” brands started releasing faster and faster films basing the assigned, or box-speed, ISO on whether they could achieve “acceptable” results at a particular speed. Whatever was the fastest speed they could get to work, not even look good, was the speed they assigned to a film. Some brands also went as far to assign an ISO so high that you could only achieve the look by pushing the film and hiding this information in the fine print on the film’s datasheet. It's created so much confusion ever since!
Box speed ISO is extremely misleading and the biggest deterrent we see at Mastin Labs for new film photographers. New film photographers will follow the film speed on the package and not get the results they hope to achieve - and that’s such a bummer because film is awesome!
At Mastin Labs we want every photographer to try film and fall in love with it, keeping film alive for generations to come, that’s why we’ve created a free chart that outlines the nominal speed of the 15 most common films available today.