The ISO setting-
Along with the exposure modes we discussed in Part I, the ISO setting is another parameter that gives you the freedom for creativity and picture quality.
You always want to shoot at the slowest ISO (lowest number) that gives you the picture you want. The ISO rating is the “sensitivity” of your camera to light, the higher the number the more light sensitive it is, but the more digital-noise you’ll introduce into your photo.

If you intentionally want a picture that blurs movement (like a stream of water as it goes by) you’d want to use a slow ISO and a slow shutter-speed. An example of this might be an ISO of 200 (the slowest ISO that the D70 is capable of) and a shutter-speed of 1/30th of a second. The camera will set the aperture (as we previously discussed) and, assuming enough light is available for a proper exposure, you’ll get the water blurred in your picture.
Of course, at such a slow shutter speed, you’d most likely want to use a tripod to avoid blurring the static areas of your photo.
Now, on the other hand, you wanted a sharp photo of a fast moving object (like a race-car) you’d most likely want to set your ISO higher than 200, more like 320 or 400 ISO and also set your shutter-speed to a higher rate such as 1/250th or 1/500th of a second.
Again, your camera would set the aperture and with the proper light, the car would be “frozen” and fairly sharp in your picture.
Other reasons to change the ISO from the default ISO 200 setting, would be in situations where there’s not enough light, and you’d like to use little or no flash. The Nikon D70 will give you a good exposure right up to ISO 1600, but for the best quality, you’d only want to raise the ISO to the lowest speed that will give you the best exposure.
Explained another way, if you can get a good shot at ISO 400 (in moderate light), there would be no need to raise the ISO to 500 or 620. The higher the ISO, the more digital-noise in your picture, and that degrades the quality.
On the D70, to change the ISO, you simply press the “ISO” button, and while holding it, rotate the MAIN Command Dial on the back of the camera.

The LCD screen on the top of the camera will display the ISO setting.
So, in practice, you want to use the Shutter-speed or Aperture you need to achieve the desired effect you want (as explained in part I), and adjust your ISO to give you your best shot.