Yes I am well aware of the basic optics, but the author is not. These headsets come with adjustable lens as well as adjustable irises; if your eye curvature was drifting a bit you’d correct with that. (And if it’s drifting a ton, increasing the depth of field is a poor fix indoors.) The dominant reason to have the iris while shooting is to help you keep three points at different distances in simultaneous focus: the target, far sight, and near sight.
