What is ISO (Film Speed)?

As a follow up to understanding aperture, I thought it would be nice to go over the basics of film speed or ISO (formerly known as ASA.)

ISO is the speed of the film, also known as the number printed on the box and the canister. FujiChrome Velvia 50 has an ISO of 50; Ilford Delta 400 has an ISO of 400.
ISO or Film Speed is expressed as a number, which is generally doubled as it gets higher. For example 50, 100, 200, 400, 800, 1600, 3200. There are several exceptions to this, such as Ilford FP4 which has an ISO of 125.
As I explained in How Film Works, photographic film is made up of millions of light-sensitive silver halide crystals, which we call grain. The lower the film speed, the finer the grain; the higher the film speed, the fatter the grain. Larger silver halide crystals have more light sensitivity than smaller ones, so a higher ISO will be more sensitive to light than a lower one. An ISO of 50 or 100 is not very sensitive and requires bright light, which means that if your camera’s fastest f-stop is 3.5 or higher, you would not have enough light to do interiors (for example) without a flash. An ISO of 3200 is extremely light sensitive, but also has very prominent grain and therefore a specific style. It’s also very easy to overexpose if shooting in direct sunlight.

This is where fine tuning the balance between f-stop, ISO, and shutter speed comes in to achieve the desired result.  When you look at a manual SLR, you’ll see a little dial with all of the film speed numbers going from 25 or 50 up to 3200 or 6400 (depending on make/model). Setting this dial is only for the light meter and will not have an effect on your exposures. Ideally, you would set the ISO, and use the light meter to balance between f-stop and shutter speed. If you’re not sure what the light meter is, it’s that little needle you see when you look through the view finder, and you want it to be in the middle for a correct exposure. My next sentence may seem obvious, but hey you never know: You need to have batteries in the camera for the light meter to work. This is why manual cameras take batteries; the batteries have nothing to do with the shutter or any mechanical part of the camera.
Here’s a little film speed guide (these are just my personal recommendations):

• ISO 50 (or lower): Bright sunlight (the beach in the afternoon, for example), studio lights
• ISO 100: Bright sunlight, bright overcast, studio lights
• ISO 200: Sunlight, overcast, some shade, studio lights
• ISO 400: Outdoor (sunlight/overcast), indoor (during the day or very well lit)
• ISO 800: Outdoor (very overcast), dusk, interiors, motion/high speed
• ISO 1600: Night, interiors (day or night), motion/high speed
• ISO 3200: Night, interiors, motion/high speed

Again, there are always ways around this, such as using an external flash head at night with an ISO of 100.

From: I Still Shoot Film Blog