ISO is the digital equivalent (or approximation) of film speed. If you remember back in the day you’d get 100, 200, 400 or 800 ISO. All of this still applies to digital photography, but it’s called an ISO instead of ASA. It’s still the same 100 and 200 for outdoors with a lot of light and 400 and 800 for indoors with less light. The same rule that the higher the number, the more light comes in still applies.
General rules for ISO settings
ISO 50, 100, 200 lowest setting depending on camera is for FULL SUNSHINE. (some high-end cameras have 50)
ISO – 400 – overcast cloudy, not so much sun.
ISO – 800 lower light – sunset time.
Noise level depends on the camera – You can see noise generally start around 800 and apparent at 1600. Your more expensive cameras have a better tolerance to noise.
The downside to using a high ISO is that digital noise or grain will appear. Below is a picture of grain. Many despise grain and avoid it all costs. Some artists use grain as an artistic effect.
Your lower end camera will produce grain at a much lower level than your full frame cameras. Lower end cameras will see grain starting at 800 and definitely visible at 1600. Your full frame cameras will go much higher before you start to show any signs of grain. high ISO settings are the biggest contributors to photographic noise. High-end cameras will pick up less noise at higher ISOs than low-end cameras, but the rule is always the same: the higher you increase your ISO, the more noise you get.
Start out by shooting outside with your camera and changing from a low number to a higher number using each increment in your settings. Watch as the picture goes from light to dark. You will need to remember the lower the number the less light will come in. The higher the number, more light will come in.