We all know about the “exposure triangle”, right? It’s one of the first things you read about when you learn the basics of photography. Shutter speed, aperture, and ISO make up the triangle, and their settings determine your exposure.
I may write an article about that someday.  (There are a lot of articles out there about that topic.  But I like to put my spin on things.)
Anyway, most of us are comfortable with using “Aperture Priority” mode and “Shutter Priority” mode on our camera. Some manufacturers use different names for these modes, but the function is the same. You set one value (aperture or shutter speed) and the camera adjusts the other value (shutter speed or aperture, respectively) to give you a proper exposure, based on the camera’s metering.
But there’s a common camera feature that many folks forget about, and that’s Auto ISO.
Most of us set an ISO value manually – typically the lowest value appropriate for the situation – and we leave it there. And that's fine. But if we’re ok with the camera setting shutter speed or aperture automatically, why not ISO?
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Manual Mode with Auto ISO
One of my favorite ways to shoot is to set my camera to Manual mode, with Auto ISO enabled. In this configuration, I have complete control over the “creative” exposure settings (the shutter speed and the aperture) and I let the camera decide the ISO. Most cameras make good decisions about this and try to keep the ISO as low as possible.
If you’re new to Auto ISO, you might be thinking: “I don’t want the camera to make my ISO skyrocket! It’ll just make noisy images!”
You’re correct to be concerned. 
Though you’re also a little too excitable – you really need to relax.
And you can relax because the Auto ISO feature on most cameras these days allows you to set a maximum ISO, thereby restricting the camera to ISO values that are at or below that value. I set mine to the highest ISO that I think my camera can handle without unacceptable noise - usually either 800 or 1600. If you look at the image below, you can see what these settings look like on the back of my Nikon.
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“Creative” Exposure Settings
I refer to shutter speed and aperture as the “creative” exposure settings because, of course, they allow you to control things like depth-of-field, motion blur, etc.
ISO, on the other hand, isn’t a very “creative” setting (unless you purposely want to introduce noise).
And the noise created as a result of higher ISO settings can be easily removed with post-production software. I’ve found that Lightroom does a great job, but there are lots of others.
When I work with a Manual + Auto ISO configuration, and I’m in a low-light or mixed light situation and shooting hand-held, I’ll typically set my shutter speed to the slowest safe speed for hand-held shots and then I’ll just adjust my aperture as needed, knowing that the ISO will take care of itself.
I check the histogram after every shot, so if I max out the available ISO, I’ll know it.
And yes, I know checking your histogram after every shot is called “chimping” and is sometimes frowned upon by some photographers.  But I’d tell those photographers to stop frowning – a digital camera gives you information, it’s silly not to make use of it.
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What About Aperture Priority Mode?
Another common approach is to use Aperture Priority mode with Auto ISO, and let the camera adjust both the shutter speed and the ISO.
I would only take this approach if your camera allows you to set a “minimum shutter speed” for this configuration (see the image above). Most cameras do allow that.  If your camera determines that more light is needed, it will adjust the shutter speed automatically (as it always does when in Aperture Priority mode), and when it has slowed the shutter speed all the way to your defined minimum, only then will it begin increasing the ISO sensitivity, up to your defined maximum.
Did you follow that?  I hope so, I don’t think I can say all that again.
The tricky part about all this is that some cameras will still drop your shutter speed below your minimum after it has maxed out the ISO and the image is still underexposed. I personally don't like that, so I don't use this approach.
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Ok, But What About Shutter Priority Mode?
Another approach is to use Shutter Priority mode with Auto ISO.  With this setup, the camera will simply adjust the aperture and ISO automatically.  The minimum shutter speed setting isn’t used.
This configuration can be useful. But of course, you’re giving up control over a very important setting – aperture – and that can significantly affect the look of your image.  But if that’s alright with you, this might be a nice way to shoot.
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So, Does Auto ISO Apply to All Situations?
No, I’m not saying that Auto ISO applies to all situations. For example, when I'm using a tripod, I manually set my ISO to the minimum and shoot in Aperture Priority, because I usually don’t care how slow my shutter speed is.  I’m using a tripod, so camera shake isn’t a concern.
And at other times, to be honest, it's not really that hard to simply change your ISO manually depending on the situation.
I find that Auto ISO is most useful while traveling when I’m shooting in a wide range of lighting situations (e.g. walking in and out of buildings, etc.) and I don't know what's coming next - so I want to be ready to shoot fast.
I would think that some wedding and event photographers might also find Auto ISO to be useful, given the fast pace of many events and the varied lighting of indoor and outdoor shots.
Street photographers, photojournalists, and anyone else that frequently needs to shoot images quickly and in varying lighting conditions could all make good use of Auto ISO.
So, when it's appropriate for what you're doing, go ahead and give Auto ISO a try. In time you’ll get a feel for what your camera can do, and you’ll start to trust its decisions.

Comments or questions?
Let me know below.
Thank you!