Mask frame styles.
A mask frame is the window on the mask which contains the lenses and come in two different styles, a single lens or two lenses. There are unique benefits of both lenses but choice of one over the other is mainly a personal preference. In certain two lens masks, you have the ability to swap out the lenses for any prescription optic corrective lenses; some also feel the single lens mask offers a wider field of vision than the two lens. Single lens masks tend to feel more natural in terms of seeing through the lens and the range of vision it gives you. Both lens types can come with an optional side window for increased visibility in which the lens extends slightly onto the side of the face for a greater side view. These different frame options are available to ensure a comfortable mask with maximum visibility.