In general you want to keep iso as low as possible. I.e., 100. As you increase iOS, the sensor becomes more sensitive to light, so that all else being equal, you can increase the shutter speed to better freeze action without blur. There is, however, a trade off. As you increase iso, your image will get grainier, as the sensor will include more noise in its image data.
So it's a trade off. As low of iso as you can get away with is generally the best answer. Auto will try do that balance for you, but it may not give you the results you want. Trial and error comes next. There is an exposure meter in the camera that you can read as you adjust iso and shutter speed. It is the EV indicator. Simply put, when you set a combination of iso and shutter speed that makes the EV indicator read 0, you should be pretty close to proper exposure. Now, variations in the subject matter can fool meters, so you may still need to tweak from there.
In a couple of paragraphs I have tried to cover a subject that is often a full chapter or more in photography books. I have left out the whole F stop portion of the equation because our Phantom camera have a fixed, not adjustable iris. Google exposure, iso, F-stop, and you will find a wealth of info out there.
Bottom line though is that there is no "best" or "correct" iso or shutter speed. It really depends on how you want the image to turn out that will dictate the settings you need to achieve your goal.
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