Depth-from-focus is an attractive approach for three-dimensional imaging owing to its compact hardware requirements and compatibility with conventional cameras. However, conventional method relies on a digital Laplacian-of-Gaussian (LoG) operator to evaluate image sharpness across a focal stack, and the required Gaussian convolution and Laplacian differentiation introduce substantial computational cost, hindering real-time operation. Here, we propose an optical analog Laplacian-of-Airy (LoA) operator that is well suited for optical implementation. To implement the key Laplacian operation, we develop an analytical scattering-matrix model for mirror-symmetric multilayer slabs, which identifies the conditions for isotropic second-order differentiation and provides a strategy for enhancing the differentiation intensity. Numerical analysis shows that LoA yields denser depth recovery than LoG in low-texture regions under sparse axial sampling. Full optical simulations further verify that the proposed optical pipeline can effectively reconstruct depth maps. By shifting the computational bottleneck from electronics to the optical front end, the proposed approach provides a practical route to high-throughput and compact depth imaging.