The parts in the left photo will slide as a unit according to the center helicoid system to make a focus. The difference between left and right can be canceled by adjusting the IPD helicoid tubes.

左の写真の部品が一体となって前後に動いてフォーカスを合わせます。　左右の視度に差があれば、目幅へリコイドで補正できます。　考え方は先のTOA150-BINOと同じです。