Nikon D80 - Why is there 2 ISO settings?

I have started photography over the past year or two and am using a Nikon D80 camera and recently I've decided to start trying to use it manually and try and get different pictures and also night pictures.

But when I've been taking night pictures I have realised my ISO was up to 1600 which isn't good it should be about 100 or 200 for night, I know how to change this but it still carried on setting it to 1600, so after a bit of playing around I realised there was ISO sensitivity set in custom settings.

I just want to know what's the difference between ISO in general menu and ISO sensitvity in the custom menu? And what should they be on once back to normal?

Also if i'm in auto mode will the iso automatically go to auto itself or not?

It is best to manually set the ISO depending upon the amount of light is available.

ISO is simply how sensitive the cameras sensor is to light.

Get out of any of the "auto modes" and use the cameras light meter to determine the proper exposure for each shot.

What you need to do is learn how to balance ISO, shutter speed and lens aperture.

You can practice that here

http://camerasim.com/camera-simulator/

ISO 1600 at night without flash is sensible. The higher the ISO value, the more sensitive the sensor gets to light. The less light you work with, the more sensitive you need the sensor to be, the higher you should set your ISO while balancing it with the amount of noise (grain). The higher the ISO, the more noise the picture gets.

I don't own a D80 but judging on what you say your ISO may be set to Auto to a prescribed range. You can change the maximum ISO value Auto ISO can go to and you can also set ISO manually if you prefer to. Read your manual on how to set ISO manually.

When ISO is set, that is the normal ISO value for that mode. There's no normal. You just have to check all settings first before you click. That's the way it is with dSLR's. If you set ISO manually in Auto mode then that will stick until you change it.

Yes, the Full Auto (green box) mode will change ISO if it isn't appropriate for the situation. This mode is only appropriate for those who can't be bothered to learn how to use a camera.

Stick with the P (Program) mode, which gives you complete control over ISO, while letting the camera adjust shutter speed and aperture.

(ISO 100 and 200 are typically too dim for night. The camera was right to increase it to 1600, although this increases image noise. If you need low ISO, you should consider using a tripod, since shutter speed will likely be increased beyond what you can hold steady.

The ISO settings are the same in either menu. Not sure why it's still shown in multiple places, but it's better than not having control at all, right?

If it's anything like my D300, the Auto ISO function is set ON by default and is NOT de-activated in manual mode! You actually have to go into camera menu settings & turn it off (I have it switched off all the time).

You should keep your ISO low as possible, even when night shooting as usually you are on a tripod then. The only time you should need to raise it is in low light levels where you ALSO need the shutter speed to be high.