The most basic difference in masks lies in the number of lenses;

Single lens masks were the first developed and remain a popular choice because they provide a broad, uninterrupted view. Single lens designs offer a clear un-obstructed view, but usually require that the lens is positioned further from the divers face this tends to restrict the horizontal and vertical vision as well as increasing internal volume.
SCUBAPRO have solved this problem with two unique solutions in a single lens design. Both the frameless and the Microframe minimise the bulk of the frame by using high frequency welding technology to join the lens and the skirt.
The end result is a single lens that sits closer to the divers face. This provide the low volume and good peripheral vision of a two lens design with the uninterrupted vision of a single lens design.

Twin lens masks are typically lower in internal volume than single lens masks and quicker to clear. By splitting the lens into two, it is possible to position the lenses closer to the divers face. This reduces internal volume and improves the field of vision. For these reasons, most sport divers use twin lens masks. SCUBAPRO's latest twin lens design optimise these advantages while positioning the lenses to prevent a blind spot in the centre.