A novel technique is disclosed for varying a size of an aperture within a vacuum chamber. A drive mechanism within the vacuum chamber is used to adjust a partial horizontal overlap between at least two blades, wherein a perimeter of the aperture opening is defined by edges of said blades. In one embodiment, a variable aperture mechanism includes first and second blades attached to a first support, and third and fourth blades attached to a second support. The first blade is spaced vertically above the second blade on the first support; a second support, and the fourth blade is spaced vertically above the third blade on the second support. There is a partial horizontal overlap between the first and third blades and between the fourth and second blades, and the aperture opening has a perimeter defined by edges of the four blades. Other embodiments are also disclosed.