How do I enable auto-ISO in non-scene modes in a Nikon D5100?

Question

I'm trying to enable auto-ISO on my dad's Nikon D5100. There is an "Auto" option for "ISO sensitivity" in the menu, logically-enough under "ISO sensitivity settings"). This works fine in the Scene modes, but with P, A, S, or M, I get:

This option is not available at current settings or in the camera's current state.

Surely auto-ISO is not only available in the preprogrammed modes. How do I enable it?

Update: there is a separate section to "ISO sensitivity settings" below the main choice. That is only available in the P, S, A, and M modes, and has "Auto ISO sensitivity control". I've set this to ON, but empirically it's the hard-set value above that actually gets used. What is this separate setting, why is it separate, and (most of all) why doesn't it work?

Answer

Auto ISO isn't available in the non-scene modes per se.

The ISO Sensitivity Auto control option you mention in the latter part of your question is essentially 'Auto Pro'. You'll note in the menu that you can set a max ISO and a minimum shutter speed.

When ISO Sensitivity Auto control is enabled, the camera will endeavour to use the ISO you set, but will increase the ISO automatically (up to your defined max) if it's not achieving the shutter speed set in the menu. If you actually take a shot then check the settings used on the LCD, you may well see that the ISO will be a different value, and shown in red, to indicate that the camera took over and adjusted it automatically.

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