Machine-tool comprising a machining module equipped with a tool-holder and a work-holder , and an optical measuring device for the three-dimensional measurement of the relative position between said tool-holder and said work-holder , said optical measuring device comprising : - an optical system with an image acquisition system, mounted on the work-holder and : - a three-dimensional target mounted on the tool-holder and comprising a useful face forming a positioning reference that can be placed in the optical axis (O) of the optical system , said useful face supporting: * a first structure defining a planar reference face divided up between at least: - a first portion whose surface is reflective according to first reflection parameters, and - a second portion whose surface is reflective according to second reflection parameters different from the first reflection parameters, and * a second structure having a face that is inclined relative to said planar reference face , and in which said optical system comprises a first exposure system and a second exposure system , in which the difference between the focal distance of the second exposure system and the focal distance of the first exposure system lies between the minimum distance and the maximum distance separating the reference face from the inclined face , in which said optical measuring device is configured to make it possible, through a single step of exposure of the target by the optical system , to determine the three-dimensional relative position between the holder of the workpiece to be machined and the tool-holder . - an optical system with an image acquisition system, mounted on the work-holder and : - a three-dimensional target mounted on the tool-holder and comprising a useful face forming a positioning reference that can be placed in the optical axis (O) of the optical system , said useful face supporting: * a first structure defining a planar reference face divided up between at least: - a first portion whose surface is reflective according to first reflection parameters, and - a second portion whose surface is reflective according to second reflection parameters different from the first reflection parameters, and * a second structure having a face that is inclined relative to said planar reference face , and in which said optical system comprises a first exposure system and a second exposure system , in which the difference between the focal distance of the second exposure system and the focal distance of the first exposure system lies between the minimum distance and the maximum distance separating the reference face from the inclined face , in which said optical measuring device is configured to make it possible, through a single step of exposure of the target by the optical system , to determine the three-dimensional relative position between the holder of the workpiece to be machined and the tool-holder . * a first structure defining a planar reference face divided up between at least: - a first portion whose surface is reflective according to first reflection parameters, and - a second portion whose surface is reflective according to second reflection parameters different from the first reflection parameters, and - a first portion whose surface is reflective according to first reflection parameters, and - a second portion whose surface is reflective according to second reflection parameters different from the first reflection parameters, and * a second structure having a face that is inclined relative to said planar reference face , <EOS>