  Optical see-through head-mounted displays (OST HMDs) typically display
virtual content at a fixed focal distance while users need to integrate this
information with real-world information at different depth layers. This problem
is pronounced in body-proximate multi-display systems, such as when an OST HMD
is combined with a smartphone or smartwatch. While such joint systems open up a
new design space, they also reduce users' ability to integrate visual
information. We quantify this cost by presenting the results of an experiment
(n=24) that evaluates human performance in a visual search task across an OST
HMD and a body-proximate display at 30 cm. The results reveal that task
completion time increases significantly by approximately 50 % and the error
rate increases significantly by approximately 100 % compared to visual search
on a single depth layer. These results highlight a design trade-off when
designing joint OST HMD-body proximate display systems.
