I got my D7100 2 days ago and spent this time setting up the D7100. My issue is the following: when I am in mode S, my shutter speed always shows 1/8000, regardless if I am in Auto ISO or not. I tried all the other modes but Manual, with or without Auto ISO and they work fine. I also tried 2 different lenses with same results. Anybody having the same problem?
I'm a bit confused by what you're getting at here. Mode S is shutter-priority mode. In that mode, you set the shutter speed yourself using the rear command dial ( or the front command dial if your custom settings are set up that way. )
When you are in S mode, you control the shutter speed directly, and Auto ISO doesn't affect it. Your camera will adjust the aperture and (if Auto ISO is on) the ISO, but not the shutter speed. That's what 'S' mode means.