When thicknesses change during tolerancing analysis, that extra distance has to go somewhere. The default in OpticStudio is to compensate with the thickness of the next airgap, but in the shop this is going to depend on the mounting arrangements of your lens. Thus, you can change how OpticStudio handles this for each individual surface to better mimic your setup. To do this, you simply add the adjustment surface number following the thickness tolerance command: TTHI Surface, Compensator_Surface. For example, TTHI 5 8 means that if the thickness of surface 5 is increased within the tolerances, the thickness of surface 8 will be reduced by the same amount to conserve the total length. If Compensator_Surface is the same as the surface in question (i.e. TTHI 5 5) the thickness will not be compensated and the total system length will increase or decrease with thickness 5.