A stereo microscope comprising: an objective assembly including an aperture; a lens; a beamsplitter; wherein the lens and the beamsplitter are configured to form a respective aperture image (512a, 512b) on each of two optical paths; the assembly further comprising, on each optical path, a stop structure (507a, 507b); wherein, the lens and beamsplitter are positioned relative to the aperture such that each aperture image (512a, 512b) is located in the plane of the respective stop structure (507a, 507b), and each stop structure (507a, 507b) blocks a portion of the respective aperture image (512a, 512b) in order to provide an exit pupil (513a, 513b), such that a stereoscopic image of an object viewed through the microscope is produceable by the combination of the images of the object visible through each exit pupil (513a, 513b); the stereo microscope further comprising: two image sensors, each image sensor being configured to capture an image visible through the respective exit pupil and to output a digital image; a digital image processor configured to apply a correction to the respective digital image output by each image sensor in order to compensate for a distortion related to the shape and position of the stop structure, the correction being based on the shape and position of the respective stop structure; wherein the correction comprises performing a spatial transformation of the respective image to correct for distortions introduced by optical structures of the stereo microscope, the spatial transformation having at least one parameter dependent on the position of the respective stop structure. an objective assembly including an aperture; a lens; a beamsplitter; wherein the lens and the beamsplitter are configured to form a respective aperture image (512a, 512b) on each of two optical paths; the assembly further comprising, on each optical path, a stop structure (507a, 507b); wherein, the lens and beamsplitter are positioned relative to the aperture such that each aperture image (512a, 512b) is located in the plane of the respective stop structure (507a, 507b), and each stop structure (507a, 507b) blocks a portion of the respective aperture image (512a, 512b) in order to provide an exit pupil (513a, 513b), such that a stereoscopic image of an object viewed through the microscope is produceable by the combination of the images of the object visible through each exit pupil (513a, 513b); the stereo microscope further comprising: two image sensors, each image sensor being configured to capture an image visible through the respective exit pupil and to output a digital image; a digital image processor configured to apply a correction to the respective digital image output by each image sensor in order to compensate for a distortion related to the shape and position of the stop structure, the correction being based on the shape and position of the respective stop structure; wherein the correction comprises performing a spatial transformation of the respective image to correct for distortions introduced by optical structures of the stereo microscope, the spatial transformation having at least one parameter dependent on the position of the respective stop structure. two image sensors, each image sensor being configured to capture an image visible through the respective exit pupil and to output a digital image; a digital image processor configured to apply a correction to the respective digital image output by each image sensor in order to compensate for a distortion related to the shape and position of the stop structure, the correction being based on the shape and position of the respective stop structure; wherein the correction comprises performing a spatial transformation of the respective image to correct for distortions introduced by optical structures of the stereo microscope, the spatial transformation having at least one parameter dependent on the position of the respective stop structure.