Human beings endeavour to see the world around them in the clearest possible manner, and may even feel uncomfortable when this is not possible. Our survival instinct demands as much information as possible to achieve physical safety. Our eyes therefore focus automatically upon every perceivable detail of a subject, and the smallest details are often required to enable the brain to assemble a spatial image from the inputs provided by two eyes. Those who suffer eye defects such as short or long sight therefore prefer to wear spectacles with correction lenses.
When viewing an image, the eyes focus in the same way to extract as much detail as possible. If the image is blurred or out of focus it may be difficult or uncomfortable to view. If a pair of stereoscopic images are soft and generally lacking in detail it will be more difficult for the brain to merge them. Sharp focus and extensive depth of field are therefore important elements of stereo photography.
In practice this means maximizing depth of field by using a small aperture - typically f/8 or smaller. With correct focus, the depth of field envelope can then be arranged to cover the whole depth of the image - from the close foreground to the far distance.