1. A method for generating a depth map for an image captured by a light-field camera, the image including at least two objects, the light-field camera including a main lens, an image sensor, and a microlens array that includes a plurality of microlenses, the method being implemented by an image processing apparatus and comprising the steps of: a) for each of the objects in the image, determining a depth level according to an angle of incidence in which light incident upon the object is projected onto the image sensor through the microlens array; b) calculating a depth value of the depth level associated with one of the objects that serves as a reference object, the depth value indicating the distance between the reference object and the main lens; c) estimating a depth value for the depth level associated with another one of the objects that serves as a relative object; and d) generating a depth map according to the depth values; wherein, in step c), the depth value is estimated based on a distance between a first location on the image sensor, on which light incident upon the reference object is projected, and a second location on the image sensor, on which light incident upon the relative object is projected.