Light field imaging can simultaneously record the intensity and direction information of rays and has the ability of depth estimation. However, the current methods only achieve disparity estimation, and few methods convert the disparity to absolute depth to achieve three-dimensional measurement. Therefore, this paper theoretically analyzes the threedimensional imaging principle of lenslet-based light field cameras and establishes the numerical conversion relationship between disparity and absolute depth of lenslet-based light field cameras. Firstly, the relationship between the disparity and equivalent baseline is obtained by the theoretical analysis of the disparity principle in light field imaging. Secondly, the imaging principle of lenslet-based light field cameras is analyzed to obtain the relationship between the equivalent baseline and the camera’s internal parameters, and the numerical conversion relationship between disparity and absolute depth is obtained by combining the disparity and Gaussian imaging formula. Finally, the validity of the conversion relationship is verified by calibrating the light field camera and building an experimental platform. Experimental results show that this relationship can convert disparity into absolute depth and obtain high-precision results.