What is ISO?
ISO is how sensitive to light the camera’s imaging sensor is. You can think of ISO like ants, at an ISO of 200 you are sending out 200 ants to gather light. At an ISO of 2000, you are sending out 2000 ants to gather light. The more ants you send out the more light you will have, so the higher the ISO the more light you will get.
Most often when people talk about ISO they talk about the noise or grain you may see when you bump it up. Digital noise is caused by low light situations. When raising the ISO on a digital camera, you are amplifying the amount of data collected which in turn amplifies the noise/grain.
Things to remember:
Lower the ISO = the darker the image, with less grain
Higher the ISO = the brighter the image, with more grain
Remember that you can help reduce noise and grain in post-processing after the fact but you can never recover subjects that are out of focus!
This chart will give you a starting point when adjusting your ISO.
If you are out in the middle of the day taking pictures, there is most likely plenty of daylight, in these cases make sure your ISO is as low as possible which will give you clean (noise-free) images. If you are inside with little light you will have to raise your ISO in order to get a properly exposed picture, which will give you a picture with a little more noise/grain. If you expose correctly the noise/grain will not be as noticeable.
For this picture, I set my ISO at 200 then adjusted my aperture and shutter speed. As the sunset, the light changed and I didn’t want to change my aperture (because I wanted a certain amount of blur) nor did I want to change the shutter speed because I was photographing kids, so I adjusted the ISO to get the proper exposure.
I will raise ISO when I can’t adjust anything else to still achieve a brighter image. When I pick up my camera I set my ISO to the lowest number I think it should be for where I am shooting. From there I will adjust my aperture and shutter speed to get the correct exposure. If I can’t get a bright image by adjusting my aperture and shutter speed I will then raise my ISO.
When testing out your ISO it is always nice to enlarge your picture to 1:1 ratio or 100% view if you are going to print it. This will help show you if your picture has a lot of noise that may be noticeable when viewing on your computer at a smaller size.
I want you to set your ISO to the lowest number based on the available light. Then adjust your aperture and shutter speed. Remembering that your shutter speed needs to be at least double the focal length of your lens. And taking into consideration what you want in focus (your aperture). Check your metering. If you can’t get the proper exposure by adjusting the aperture or shutter speed then increase your ISO.
Check out Determining your Maximum ISO to help you determine the highest ISO you are comfortable shooting within an average situation.