An assembly, comprising: a mid- infrared objective lens assembly for gathering mid-infrared light from an object positioned at an object plane and forming the mid-infrared light at an image plane, the objective lens assembly comprising: a plurality of spaced apart, refractive lens elements that operate in the mid-infrared spectral range, the plurality of lens elements including an aplanatic first lens element that is closest to the object, the first lens element having a forward surfaces that faces the object and a rearward surface that faces away from the object, wherein the forward surface has a radius of curvature that is negative, wherein the first lens element has a working distance of less than one hundred millimeters; wherein the plurality of lens elements are spaced apart along an optical axis, wherein the forward surface of the first lens element has a forward center of the curvature; and wherein the first lens element is positioned so that the forward center of curvature is positioned on the optical axis, and is approximately at the object plane; wherein the forward surface is spaced apart from the object a front separation distance (a) along the optical axis: the assembly further comprising: a mover assembly (25B,76) that moves at least one of the object and the front lens element to adjust the front separation distance; and a control system that controls in a feedforward fashion the mover assembly (25B, 76) to selectively adjust the front separation distance based on the wavelength of the beam transmitted through or reflected by the object to reduce RMS wavefront error. a mid- infrared objective lens assembly for gathering mid-infrared light from an object positioned at an object plane and forming the mid-infrared light at an image plane, the objective lens assembly comprising: a plurality of spaced apart, refractive lens elements that operate in the mid-infrared spectral range, the plurality of lens elements including an aplanatic first lens element that is closest to the object, the first lens element having a forward surfaces that faces the object and a rearward surface that faces away from the object, wherein the forward surface has a radius of curvature that is negative, wherein the first lens element has a working distance of less than one hundred millimeters; wherein the plurality of lens elements are spaced apart along an optical axis, wherein the forward surface of the first lens element has a forward center of the curvature; and wherein the first lens element is positioned so that the forward center of curvature is positioned on the optical axis, and is approximately at the object plane; a plurality of spaced apart, refractive lens elements that operate in the mid-infrared spectral range, the plurality of lens elements including an aplanatic first lens element that is closest to the object, the first lens element having a forward surfaces that faces the object and a rearward surface that faces away from the object, wherein the forward surface has a radius of curvature that is negative, wherein the first lens element has a working distance of less than one hundred millimeters; wherein the plurality of lens elements are spaced apart along an optical axis, wherein the forward surface of the first lens element has a forward center of the curvature; and wherein the first lens element is positioned so that the forward center of curvature is positioned on the optical axis, and is approximately at the object plane; wherein the forward surface is spaced apart from the object a front separation distance (a) along the optical axis: the assembly further comprising: a mover assembly (25B,76) that moves at least one of the object and the front lens element to adjust the front separation distance; and a control system that controls in a feedforward fashion the mover assembly (25B, 76) to selectively adjust the front separation distance based on the wavelength of the beam transmitted through or reflected by the object to reduce RMS wavefront error. a mover assembly (25B,76) that moves at least one of the object and the front lens element to adjust the front separation distance; and a control system that controls in a feedforward fashion the mover assembly (25B, 76) to selectively adjust the front separation distance based on the wavelength of the beam transmitted through or reflected by the object to reduce RMS wavefront error.