The optical see-through (OST) display is one of the key enabling devices for augmented reality. Despite the latest craze such as with the Google Glass, there are still many ergonomic problems associated with the OST displays. One of the already well known such problem is the “refocusing” problem, in which the user has to switch one’s focus between the distant real world and see-through display up front. Such refocusing, for one, is bound to cause significant strain and fatigue to the eyes. However, there are not many studies, nor guidelines devoted to this issue. In this preliminary work, we ran experiments to measure the degree for eye strain and its pattern at different refocusing distances and durations (or number of focused targets). The findings should serve as one guideline in designing OST glass based interaction and applications.