What is ISO film speed? How does it interact with time of exposure?

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What is ISO film speed? How does it interact with time of exposure?

In: Technology

Iso stands for international standards organization. They set the iso rating for film and what the numbers mean. Lower numbers mean slower exposure meaning they need to be exposed longer than faster (higher) numbers. I believe the relationship is linear, so ISO 100 will take twice as long to properly expose compared to ISO 200.

ISO represents the “sensitiveness” of the film (or sensor).

Now think of exposure like completely filling a glass of water. Here, water acts like light in photography:

* The faucet nozzle is the Aperture: its size controls how much water can flow into the glass in the same way aperture controls how much light “flows” to the sensor (or film).
* The time your faucet is open is the Speed: the more you keep it open the more water (light) can flow.
* The glass size is the ISO: a smaller glass will be full in less time or in the same time but with a smaller nozzle. In the same way a more sensitive sensor (or film) will be exposed correctly in less time (or using a smaller aperture and the same time)

ISO is an indication of sensitivity to light. Higher ISO=higher sensitivity. In film, higher ISO means more light sensitive chemical. In electronic cameras, it means higher amplification. That leads to greater noise in the image. The shutter time and aperture also contribute to the exposure.

It’s how sensitive the film/sensor is. The higher the value, the more sensitive it is, the faster it “burns”/registers light.
This means it allows for shorter exposure times and/or narrower (higher value) apertures.
Higher sensitivity means less precision though, and increasing ISO increases noise in the image.

Attempt at true ELI5:
Imagine hot air coming through a mesh. Low ISO is a finer mesh, air barely comes through and you need to wait for a bit to heat the room with it.
When you increase the mesh size (increase ISO) more air comes through, and the room gets air almost immediately, but it also lets dust and other debris in.

PS: image noise isn’t “light imperfection”, it’s a chemical phenomenon in film, and amplification artifacts in digital, but for simplicity’s sake…