How to take pictures using ISO?

I'm getting a Nikon DSLR and I want to know when I'm taking pictures outside and inside what ISO number should I use? What number should I use when their is less light? And when I'm zooming onto a bird such as 85mm away from what ISO should I use?

I always use the lowest ISO I can while still avoiding motion blur. If I can't make a sharp picture then I up the ISO or use a tripod.

You will want to use the lowest ISO possible for the light conditions in an effort to defeat digital noise. There are three factors that affect your exposure. ISO is one of them. The other two are aperture and shutter speed. As one of them changes, one or both of the other need to be adjusted to keep the same exposure.

ISO higher number more sensitive to light so less light is needed for the same exposure
Aperture lower number means larger opening so more light coming in in the same amount of time
Shutter speed slower shutter speed (lower number) the longer the shutter stays open so the larger the amount of light gathered from the same opening

Typically, you decided whether you are concern about depth of field (and so adjust aperture) or amount of action frozen (and so adjust shutter speed). From there, you adjust the ISO for a good exposure.

The best ISO for general shooting is ISO 200. I don't usually use the -1 range, where it goes down to 100. Stick with ISO 200, unless the light drops, or you want to create an effect photo (extra grain, super high speed, etc.)

When the light gets lower, your limiting factor is your own sense of what a quality photo should look like. This depends on the camera and the sensor. I have a D300, and I quit shooting at ISO 800 if it's something I care about, ISO 1600 if it's for fun, and ISO 3200 if I don't care about how it looks.

A D3 can shoot up to ISO 6400 and look pretty good, a D3S can go up to 25600 and still produce decent results.

If the bird is 85mm away, I'd think more about ducking than shooting. Do you mean 85 meters away, or zooming to 85mm with your lens?

It doesn't really matter in either case, the available light dictates the ISO, and the subject dictates the shutter speed. If the bird is sitting in a tree, then you can use 1/60 of a second, if it's flying straight across, then 1/500 of a second. If it's bright sun, you use ISO 200, if it's just after sunset, you probably can't get a shot that will be usable, ISO will be too high, shutter speed too low.

And for birds, most people are using a 300mm lens as a minimum, the super telephoto lenses are preferred (500mm, 600mm, 800mm)

Use the lowest ISO you can for the ammount of light you have while allowing for a fast shutter speed.

If your shutter speed is too low and you already have increased your aperture to try and compensate as much as you can then consider that each level of ISO is one step of light… Same happens with shutter speed.

For example. If you are shooting at 1/30th at 100 ISO but you need to shoot at 1/50th you could increase your iso to 200 to keep the same exposure.