Previously, we talked about how shutter speed and aperture affect an image. The third main setting you have to worry about is ISO sensitivity.

Your digital camera creates an image by processing the light that comes into contact with its internal sensor. The ISO sensitivity is a measurement of how sensitive the sensor is to that light. Jack it up, and it will magnify the intensity of the light. Turn it down, and it will minimize it.

What Happens When You Turn Up the ISO Sensitivity?

By turning up the ISO sensitivity, you artificially increase the amount of light captured by your camera’s sensor. This effectively makes the image brighter. For example, changing the ISO from 400 to 800 will effectively double the amount of light captured by your camera.

There is, however, a trade-off. The increased sensitivity to light also means an increased level of noise – little speckled dots that make your picture look… not so good.

The image to the left was shot at 3200 ISO, which is pretty high. The image also came out under-exposed, so I had to brighten it in Lightroom (which also magnifies the noise).

So Why Use a High ISO Rating?

There are three basic things that affect an image’s exposure – shutter speed, aperture, and ISO. Let’s say you’re shooting a sporting event, like the track meet where the picture above came from.

You need a relatively high shutter speed. In this case, I picked 1/160, which isn’t ideal but it’s fast enough.

This also means you’ll need to open up the aperture and let in a lot of light. The maximum width of the aperture depends on your lens, though. At this time, I still had a basic zoom lens that had a wide open aperture of f/5.6.

With those settings and an average ISO of 400, the picture was really under-exposed. I couldn’t lower the shutter speed (for fear of more motion blur), nor could I open up the aperture (because I hit the physical constraints of my lens). My only option was to increase the ISO until the picture was relatively well exposed. I didn’t want the noise, but it was a necessary evil to get the image to come out at all.

So What Are Average ISO Ratings/Uses?

Low ISO. For most cameras, this is 100 to 400. This is great for shooting outside where there’s a lot of light, or when you’re using lights to artificially light your subject. Either way, you don’t need to worry about light, so there’s no need to artificially increase it.

High ISO. This can range from 800 up to crazy levels like 12,800 (not recommended). You’ll often need this for indoor or night sporting events where there is limited light and you still want to maintain a quick shutter. If you’re shooting sports outdoors on a cloudy day, you might also need a medium to high ISO (400 to 800).

As a general rule of thumb, you want to pick your shutter speed, pick your aperture, and then adjust the ISO accordingly.  You want to use the lowest ISO possible, to avoid noise, but if you can’t bring in more light with the other two settings its a necessary evil.