According to one embodiment, an optical device includes a variable aperture mechanism configured on a structure having a radiation detector that receives radiation through an aperture of the variable aperture mechanism. The aperture is selectively movable from a first position to a second position in which the aperture has a different size relative to the aperture in the first position. The structure is configured with one or more magnets that function with one or more magnetically permeable members configured on the variable aperture mechanism to hold the variable aperture mechanism in at least one position using a magnetic force between the magnets and magnetically permeable members.