Camera film speeds (ISO 100, 200, 400 etc) when to use them?

Film speed Examples of use
ISO 100
“slow film”
Anything brightly lit: Inside when a flash is needed, or outside when there’s bright sunlight.
ISO 200
“medium film”
General purpose: Indoors when you might need a flash, outside when there’s decent light.
ISO 400
“fast film”
When there’s less available light: Daytime when it’s not so sunny, dusk/dawn, indoors without a flash. Or for when subject is moving at speed (will cut down on blur).
ISO 800, 1600
“fast film”
Low light conditions: Indoors without a flash, subject moving at speed.

On old cameras that used actual photographic film, the film was made to suit different speeds. You’d also adjust the shutter speed on the camera with the f/stop. In digital cameras it just refers to light sensitivity: The higher the number, the more sensitive it is.

A slow film speed (like ISO 100) is less sensitive to light than other film speeds: It requires a longer exposure to light to make a picture. The advantage of slow film is that it gives you a much sharper image, so you should always use the slowest film speed possible.

A fast film speed (like ISO 800) is more sensitive to light, so can be exposed for a short time to make a picture. It’s good for low-light conditions or subjects moving at speed, but can be grainy. It will give you a brighter image than the 100 film speed in low-light conditions, but will look over-exposed on a bright sunny day outside.

The ISO numbers refer to the amount of time it takes to capture an image, and this time doubles with each increment, so 800 film would take 1/8th of the time that ISO 100 film would need to capture the same image. Or to put it another way, ISO 800 speed film is eight times more sensitive than ISO 100 speed film.

Why I wanted to know this?
We have an old (like ten year old) digital camera, it was top of the range when we bought it and is holding up well, but does suffer from “over exposure”. You can’t quite use it as a point-and-shoot idiot camera: One of the things you have to choose is a film speed: I’m hoping using the above will help cut out some of the “over exposure” problem…

Note: “ISO” seems to stand for “International Organization for Standardization”, a film speed standardisation from 1979 (Wikipedia article here).

Sources:
Understanding Film Speed by Frank Logue
Fuji Film
Photography Life

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