The distance between the optical axis and the center of the OAG prism is 17.5mm. This distance provides the largest possible vignetting-free field while retaining all the advantages of the large and movable prism. This information is important, among other things, to be able to estimate or avoid vignetting of the camera sensor. If the prism protrudes too far into the light path, an included 4mm spacer ring can be inserted. However, this reduces the usable area of the prism, and thus also the amount of light that can be used for tracking.