The meaning of depth of field is the distance between the nearest        and the furthest objects that give an image judged to be in focus in a camera. The distance between the camera and the first element that is considered to be acceptably sharp is called DoF near limit.  The way it zooms  , the further away you are from a subject the larger the depth of field . Focal Length – the shorter the focal length the larger the depth of field. So with a zoom lens you have multiple focal lengths throughout the zoom range and so yes zooming in and out also affects the depth of field . Depth of field    is controlled by changing the aperture setting on your camera. Like your eye, a camera lens has an iris inside that can open or close to let in more or less light. You control the size of this hole, or aperture, by changing the aperture setting, which is measured using a scale of f-stops. Using the aperture (f-stop) of your lens is the simplest way to control your depth of field as you set up your shot. Using a shallow depth of field is a good way to make your subject stand out from its background and is great for portrait photography. Larger apertures (smaller F-stop number) and closer focusing distances produce a shallower depth of field,  depth of field only sets a maximum value for the circle of confusion ,