The depth of field (DOF) of an auto-stereoscopic display refers to the depth range in 3D space in which objects can be depicted with small amount of blur. It provides a measurable index on the display's performance in reproducing light fields of 3D scenes. Previous studies have analyzed the maximum spatial frequencies of aliasing-free images depicted on planes parallel to the display's surface. For multilayer displays, several formulae representing the upper bounds on the maximum frequencies have been given. However, these formulae provide little information on how much blur would be present in the reproduced fields, since contributions of low frequency signals are simply neglected. Such signals are frequently damaged on multilayer displays especially when the angular range of viewing angles becomes wide. To address these drawbacks, we present a novel framework for the DOF analysis of multilayer displays. The analysis begins with a close look at the synthesis of layer images, which can be considered as solving a linear least squares problem with nonnegativity constraints. This numerical procedure is then reinterpreted in the context of multilayer displays, where part of the connections between "depth" and "blur" are observed. Finally, experimental results supporting these observations are presented.