A collapsible quad chair has a frame selectively moveable between a collapsed configuration of the chair and a use configuration of the chair, and a fabric frame cover attachable to the frame to define a back portion and a seat portion of the chair. The method and system include sensing LV activation events at multiple LV sensing sites. From the relationship between the obtained picked-up image of the mark and the defocus amount, the positional information of the mark is estimated. A release mechanism allows disengagement. Preferably, at least one of the zones has a power which is more negative than a zone of the same character which is situated closer to the center of the lens. The reflecting type light valve forms a part of the partition wall, and the light incidencereflection surface faces the light incidencereflection side and a part opposite to the light incidencereflection surface of the reflecting type light valve is cooled by the cooling system.