ISO is the one camera setting you can change without affecting your image looks too much, at least for lower values. At higher values, visible digital noise can become an issue.
RELATED: Your Camera's Most Important Settings: Shutter Speed, Aperture, and ISO Explained
The Default: Your Camera's Base ISO
Every camera has a base ISO. This is the baseline sensitivity of the sensor, and it's the value at which it operates best with the highest dynamic range.
For the vast majority of DSLRs and mirrorless cameras, the base ISO is 100, although a few high-end Nikon cameras have ISO 64.
ISO is not necessarily the lowest. For example, my Canon 5D III has ISO 50 setting, but this is achieved by reducing the gain on the sensor.
Since you get the highest quality images at the ISO, it should be use it. If you can get the shutter speed you want with ISO 100 (or ISO 64, check your camera's manual to be sure), then that's what you should use.
Note: The image above was shot on a Canon 650D at ISO 100.
The Sample Images for each ISO value are cropped versions.
Digital cameras are incredible. They have come out in leaps and bounds over the years, and there is no doubt that they can not do so between ISO 200 and ISO 800 with almost no discernible drop in image quality
If you need to use a faster shutter speed or narrower aperture than your base ISO will allow you to confidently increase the ISO to around 800 without having too much of an effect on the image. I regularly shoot portraits at ISO 400 so that I can guarantee my shutter speed does not drop too low.
I'm Kind of More Arbitrarily Calling ISO 800 the top of the range because of it's as high as some entry in the field of quality cameras, but some newer and full-frame cameras, you'll be able to push it higher. The best thing to do with your camera and how it operates at different values.
Somewhere between ISO 800 and ISO 3200 range, you may not see it at all. Again, it's kind of camera-specific;
This range may not be a better endeavor than any other camera.
Increasing ISO to this point is a tradeoff. You're almost certainly shooting at night or somewhere dark and, if you can not reduce your shutter speed or widen your aperture any more, then the ISO is your only option. In this range, you're still going to get good images, but they just do not want the highest quality. Still, a good photo is better than no photo.
ISO 6400 and Beyond
Once you start to push past ISO 3200, you want to see a dramatic increase in noise. As always, the exact value depends on your point of view, the images will become unusable, at least for professional contexts.
I did a series of night portraits at high ISO values and because I embraced the noisy look, I was able to shoot them at ISO 6400 without worrying too much.
On the other hand, if you 'going back for a super clean look, then you're probably out of luck.'
The other option is to look at other ways of reducing noise. Astrophotographers regularly shoot multiple photos at ISO 6400 and then combine them in post-production to offset the noise from the other images.
ISO is the first place to go when you need to increase exposure, and that's fine-to-a-point. Once you see a visible decrease in image quality, you need to start thinking more carefully.