What is it?
We made it! A digital photograph does not exist until your camera's image sensor is exposed to light. It's the place where all those mysterious megapixels you were sold on live. Quite literally, your image sensor is a rectangular-shaped platter of millions of little light sensitive pockets called pixels waiting for you to snap your shutter so they can collect light, divide it into red, green and blue and save it as a file on your memory card. All that to say - the image sensor is what receives the light and interprets it into an actual digital photograph.
How is it measured?
ISO is the measurement of how sensitive your camera's sensor is. Why it's called ISO isn't going to help our cause, so don't stay up at night worrying about it.
Depending on your camera, ISO numbers are typically 100, 200, 400, 800, 1600, etc.
If you remember anything about film cameras, ISO is very much like film speed. Changing this setting doesn't actually change how much light your camera collects (that's up to the aperture and shutter), it just tells your camera how sensitive it needs to be in receiving it. A low ISO setting means the sensor is less sensitive, while a higher ISO setting makes your sensor more sensitive.
What does this mean to you?
When there's lots of light, there's no reason to increase the sensitivity of your image sensor. So on a bright sunny day - set ISO to it's lowest available setting. As the light decreases (shade, evening or darker interiors) increase your ISO. That's it - easy peasy!
Your camera was optimized to make the best images when ISO is set to it's lowest setting. If the right amount of light is let through the shutter from the aperture, a low ISO setting allows the image sensor to deliver it's best work - crisp, clear, pristine digital images. So, if you can, you want to keep it low.
Sometimes, especially indoors, there just isn't enough light available for your sensor to get a good exposure. Turning up the ISO increases your sensor's sensitivity, making more out of the light it receives.
But NOTHING comes free, right? The higher you turn up the sensitivity, the less accurate the sensor becomes. What's referred to as "noise" is introduced to the image.
Look at the photo above. The ISO on a beautifully lit late afternoon was accidentally left on 1250 when it probably should have been closer to 200-300 (hey, it happens to all of us!), and what would have been a gorgeous set of photos was marred with noise. But on the shots where the exposure was pretty good, the noise wasn't that big of an issue, even at full magnification. You can see some of the telltale dots, especially in her hair and in the shadows on her skin, but unless this photo was meant to be blown up, it really is fine for the family collection.
Where we run into trouble is in the photos where the exposure wasn't correct. Normally we'd just adjust the exposure in the computer after the fact, but photos with noise resulting from high ISO settings don't respond very well to even minor adjustments in post proccessing. Below is another photo taken on the same afternoon with the same unfortunate ISO settings. A beatiful moment was captured, but underexposed. Look what happens to the noise when we try to correct the exposure - the noise practically jumps off the screen!
You don't want noise in your photos, but unless the only place you take pictures is in the bright outdoor light, raising the ISO is, at times, unavoidable. The good news? Camera sensors are getting more and more accurate at higher ISO levels, especially when you get the overall exposure correct. Need more good news? Even pictures with a lot of noise are fairly pleasing at smaller sizes (see the picture above on the left). So no matter what, never sacrifice missing a moment because you were afraid to turn your ISO too high! Go for it!
A low ISO setting is ideal. But nothing's perfect and you'd be much better off raising it and getting some noise than missing your toddler's first sommersault because the lighting at the gym was less than ideal. (oh, trust us, lighting at the gym STINKS!)
Low ISO number = use in BRIGHT setting = lower sensitivity and less noise
High ISO number = use in less light = higher sensitivity and more noise