Apparatus for increasing resolution of cryogenically cooled vacuum-sealed infrared imaging detector. A two-dimensional detector array of photosensitive pixels is successively exposed to an image scene, acquiring multiple imaging samples. A masking filter, disposed between the detector array and image scene focal plane, is maintained at a fixed position with respect to the detector array, and reduces the region of pixels collecting incident radiation for each imaging sample such that only a portion of each pixel area of the image scene is imaged onto the corresponding detector array pixel. Shifting means successively shifts the optical path of the image scene relative to the masking filter, by a shifting increment or fill-factor reduction amount equal to a fraction of the array pixel width, to image different sub-pixel regions in each imaging sample. A processor reconstructs an image frame having a resolution greater than the intrinsic detector resolution by the fill-factor reduction amount.