There are times when analysis of the MTF along some arbitrary orientation is required. Not in rotationally symmetric systems, of course, but in some off-axis systems. For example, it is possible to get a 4-lobed OPD, and it it may be important to check the MTF along both the tangential and saggital fans, as well as +/- 45 orientations at the same field point.
MTF is the most common metric for this orientation sensitive performance analysis, and we want to maintain maximum flexibility for all cases. Technically, this was possible using vignetting angles in previous releases, but this wasn't clearly stated. So, in the OpticStudio 17 release, we're updating the documentation and naming conventions from "Vignetting Angle" and VAN to instead say "Tangential Angle" and TAN. These tangential angle settings can be used to rotate the orientation of the MTF calculations as well as the new Contrast Optimization
For more information on how to use the tangential angle settings, please see this video: