What Does This Button Do? - ISO So What?

So this is the one I paid attention to when I shooting on film and stopped being aware of when I went to digital.

What is ISO?  ISO is the sensitivity of your film or sensor.  When you shot film you were stuck with one setting on each roll of film.  So if you put in a roll of 100 ISO film for shooting outdoors, but didn't use it all before needing to shoot indoors or in low light, you were kind of hosed.  The great thing about digital cameras is the ability to change settings on a shot-by-shot basis.

A low sensitivity, like 100 or 200 is great for outdoors, in bright sunny circumstances.  You can still shoot handheld with a quick shutter and narrow(ish) aperture and get a nice clear shot.  Low ISO produces cleaner, clearer pictures.

High ISO lets you shoot with a quick shutter and/or narrow aperture, but has the unfortunate effect of dark areas becoming 'grainy' or 'speckled', especially when viewed at full size.

Here's an example:
This is a piece of the tulip photo from the first Navy Pier Flower and Garden Show post.  This shot was done at 100 ISO so that even the dark spot at the tip of the anther looks smooth, color-wise.

In contrast, this a piece of a shot of a rose done at 800 ISO.

You can see that, especially in the background where it's dark, the colors don't blend seamlessly into each other or have nice, stark delineating lines.  There's a grainy look to the photo.

ISO is a balancing act.  If you're shooting outdoors, say on a nature hike, or indoors while your three-year-old runs around at his birthday party, you may not have the luxury of being able to stay at 100 because when your ISO is that low the camera will compensate by leaving the shutter open longer and opening the iris more, exposing the camera to more shake or to movement of the subject.  Ideally you want to keep your ISO as low as your lighting will tolerate. If you're using a tripod and a still subject, go all the way down.  If your subject is moving or your light is low you may have to creep it up to 400 or 800.  Once you get higher than that you can almost be certain there will be a fair amount of grain.  Now, if you're shooting urban, 'gritty' scenes, you can make the grain work for you and have it be part of your style choice, but if you want clean, sharp images, go as low as you can.

Most cameras only allow you to chose your ISO in Av, Tv or Manual mode.  Many of the presets will automatically chose a higher ISO to keep your shutter time down.  I use what I call "Stuff's-moving" mode (otherwise known as 'sports mode') a lot when shooting wild animals and unless it's very, very bright out my camera tends to default to 800 on that setting, but sometimes when I'm trying to get squirrels playing or wildflowers blowing in the breeze, it's a compromise I have to make since running a longer shutter would give me an unacceptable amount of blur.

One time I use 800 ISO is when shooting fireworks.  You can Google the settings that will work best for your particular camera - some even have a fireworks preset - but when you want to get the little details like the trails of light in the finale, a high ISO is good thing.