Depth Of Field Button On D70
Use the viewfinder to view and focus on a close subject. At this moment you will be viewing the subject with the iris of the lens set wide open and set to the largest aperture. This situation will provide a bright viewfinder image and a very small depth of field with most of the subject in front and behind the focus distance being out of focus.
When you depress the Depth-of-field preview button the iris of the lens will be stopped down to the aperture that the cameras meter has selected for the exposure. At this moment the image in the viewfinder will go dark because less light is entering the viewfinder but if you look carefully you will also see a greater depth of field and you can now determine how much of the picture will be in-focus. To increase the effect use PAS or M modes to select a smaller aperture (i.e. higher F number such as f/22).