A thermal infrared camera may be used under a wide variety of target-scene radiation conditions, with interchangeable or zoom lenses requiring matching or different size cold stops. A variable aperture assembly of a thermal infrared camera integrates a rigid open truss-like framework that's capped by an aperture ring and bottomed by a driving ring, and a radiation shield, located inside the framework, that contains an aperture ring at an upper side. A plurality of blades that collectively define an aperture positioned between the upper aperture rings. Opposite blade ends are coupled to respective ones of the two aperture rings, permitting pivotal movement in one ring and radial movement in the other ring, when the rings are rotated relative to one another, to change the size of the formed aperture. Both refractive and reflective infrared telescopes may be retro-fitted with variable aperture devices to enhance infrared imaging performance.