Manual or auto ISO for photos?

[msinger]

When taking photos outside on a normal sunny day, is it best to set the ISO setting to 100 or leave it at auto?

 

[4wd]

If it's sunny it would likely default to 100 anyway, but noise gets significantly worse as ISO increases so if you have half decent light definitely best locked on 100 as a rule.
There's a chance that doing so will slow shutter speed to compensate (letting in more light) and eventually that could result in motion blur as light levels fall, especially if the phantom is not practically stationary