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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?

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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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Nikon's Auto ISO works in a different way in Scene/Auto modes than it does in P-S-A-M. In Scene modes, if you get to the lowest native setting (100 on the D5100), you can scroll once again "below" it to get to the "Auto" position. In P-S-A-M, however, you don't get the extra position and have to enable it in the menu, but it still respects the chosen value.

Let me illustrate it with an example. Say you're in Aperture Priority and set a wide aperture (e.g. f/2) and a higher-than-base ISO (e.g. 800). Up to a certain point, the camera will shoot at the chosen aperture with the set ISO and will adjust the shutter speed accordingly. If, however, the camera detects that it would overexpose the shot even when it reaches the highest available shutter speed (1/4000 for the D5100), it would lower the ISO to a value that would allow a correct exposure. So that's A 1/4000, f/2, ISO 800 normally, but when the camera detects a possible overexposure by one stop, this will become 1/4000, f/2, ISO 400. Somewhat the same thing happens on the other end - if you hit the lowest shutter speed that's set in the menu (e.g. 1/60) and the highest ISO (e.g. 6400) and the camera still thinks it would underexpose the shot, it will attempt to drop the shutter speed, so what normally is A 1/60, f/2, ISO 6400 may become 1/30, f/2, ISO 6400.

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There is no such story to worry about it can only happen when your camera's battery is out of charge,Your camera setting will be back once you recharge. All what you have to do is go and recharge your battery rather than, changing your camera. Thank you

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    Are you saying that auto-ISO works when the battery is full but not when it is low? That seems... unlikely. – mattdm Feb 22 '18 at 4:51

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