With the recent development of plenoptic cameras, light field imaging has become a reliable solution for 3D metrology. Plenoptic cameras allow to recover the 3D information of a scene with a single acquisition and only one camera. These advantages make light field imaging particularly interesting for the metrology of two-phase flows. The measurement depth of a back-light imaging set-up can be determined by the calibration of the Point Spread Function (PSF) of the optical system. This study provides a method, adapted to multi-focus plenoptic cameras, to measure the PSF width from reconstructed images. We used a calibration target composed of multiple calibrated opaque disks and a diffuse backlight illumination set-up. The PSF width is estimated by measuring the grayscale gradient at the interface of the images of the disks. Coupled with the depth information provided by the plenoptic camera, we obtained the evolution of the PSF at different depth planes. The information on the PSF enables to obtain the focus criterion which sets the limits of the measurement volume. The spatial resolution of the reconstructed images is obtained from the PSF. The results obtained for the resolution as a function of virtual depth are consistent with results from previous studies.