Along with shutter speed and aperture, ISO is one of the three fundamental parameters for determining the correct exposure of your images. ISO is a bit different from aperture and shutter speed, however, as you almost never use it for creative purposes like the other two. So, how do you manage it properly? This helpful video tutorial will show you what you need to know to choose the right ISO for every photo.
Coming to you from Dylan Goldby, this excellent video tutorial will teach you how to choose the right ISO for whatever photos you are taking. By far, the biggest mistake I see photographers make with ISO is being afraid to raise it out of fear of noise and keeping the shutter speed too low for their focal length or the action in the frame as a result. You can always do some noise reduction on a sharp but noisy photo in post (and modern software is remarkably good at this), but on the other hand, there is not much you can do to save a photo that has blur from camera shake or subject motion. Don't be afraid to raise your ISO if it'll get you the shot you need. Check out the video above for the full rundown from Goldby.
"the biggest mistake I see photographers make with ISO is being afraid to raise it out of fear of noise and keeping the shutter speed too low"
Yeah, as an event photographer working a lot in very low light, I had to learn this one the hard way.
Now, though, I am using ISO-invariant cameras (Sony a7RIII), so pretty much any ISO 800 and up is "correct" in dim conditions. If an image seems too dark, I can brighten it in post by up to about 5 stops without any penalty to noise or detail as compared to using a higher in-camera ISO setting. So, I set an action-stopping shutter speed and an appropriate aperture manually, and enable Auto ISO, with Exposure Compensation dialed down -1 or -2 stops to preserve highlights. Then I brighten to taste in post. The one downside is that, because my JPEGs come out dark, I can't hand them off onsite. But, I almost always deliver finished JPEGs processed from RAW within 48 hours anyway.