Wouldn't you want shutter priority mode?
Learn something new every day, I didn't realize there was a minimum shutter speed setting in the Auto ISO menu. I've actually never used Auto ISO, but it sounds like something I may need to look in to checking out.The idea is that we always have to keep a watch on all of the ISO, aperture, and shutter speed values.
Auto ISO only works (only increases) when the normal hardware reaches its physical limits. If not hitting a limit, no Auto ISO increase. Auto ISO is for when all else fails.
If we use shutter priority, the aperture will always be open wide open before Auto ISO starts to increase. Probably not the best plan, unless you specifically want wide open aperture. But in any dimmer light, Shutter priority will absolutely insure wide open aperture.
Likewise, if we use aperture priority, we can set the aperture we want, but then shutter speed can fall to the lowest shutter speed. But we avoid that being the actual 30 seconds hardware limit ... with the clever Minimum shutter speed set in the Auto ISO menu, which is intended to be the actual Auto ISO threshold. We can set that to be 1/500 second as stated. Then if shutter becomes as slow as we set that limit (the 1/500 second), then the dropping shutter speed holds there, and then Auto ISO increases. Shutter speed can of course still go slower when we hit the Maximum ISO limit.
We do still need to realize two things... 1) Any Auto ISO value between Minimum ISO and Maximum ISO will use this Minimum shutter speed we set, so we need to give it some thought.
And 2), things can still change, and we always need to keep a constant watch on everything.
I thought it was a surprisingly good article.. Much more meat than we normally see on the internet.