An optical system for capturing an image of an object, the optical system including a high NA (numerical aperture) objective lens for collecting light from the object, wherein said high NA objective lens has a numerical aperture in air of at least 0.5, a focusing apparatus configured to adjust the focus of the high NA objective lens, and a light detection system including a detector and focusing means adapted to focus the captured image onto the detector, the focusing apparatus including: an image forming and capturing means (26, 28) configured to receive light from the high NA objective lens, form an image of the object by focusing the received light in an intermediate image zone, and capture an image of the object in the intermediate image zone by receiving and refocusing said received light from a selected focal plane at a position within said intermediate image zone, and a focus adjusting means configured to adjust the position of the selected focal plane within the intermediate image zone, wherein said image forming and capturing means includes at least one high NA (numerical aperture) focusing lens that has a numerical aperture in air of at least 0.5 and a mirror, wherein the focus adjusting means is configured to adjust the relative axial positions of said at least one high NA focusing lens and said mirror, and wherein the at least one high NA focusing lens is optically matched to the high NA objective lens, and the system being characterized in that: the intermediate image zone is located between the at least one high NA focusing lens and the mirror, and the at least one high NA focusing lens is configured to receive light from the high NA objective lens and form the image of the object in the intermediate image zone, and the mirror is configured to receive said light from the intermediate image zone and reflect it back to the at least one high NA focusing lens. an image forming and capturing means (26, 28) configured to receive light from the high NA objective lens, form an image of the object by focusing the received light in an intermediate image zone, and capture an image of the object in the intermediate image zone by receiving and refocusing said received light from a selected focal plane at a position within said intermediate image zone, and a focus adjusting means configured to adjust the position of the selected focal plane within the intermediate image zone, wherein said image forming and capturing means includes at least one high NA (numerical aperture) focusing lens that has a numerical aperture in air of at least 0.5 and a mirror, wherein the focus adjusting means is configured to adjust the relative axial positions of said at least one high NA focusing lens and said mirror, and wherein the at least one high NA focusing lens is optically matched to the high NA objective lens, and the intermediate image zone is located between the at least one high NA focusing lens and the mirror, and the at least one high NA focusing lens is configured to receive light from the high NA objective lens and form the image of the object in the intermediate image zone, and the mirror is configured to receive said light from the intermediate image zone and reflect it back to the at least one high NA focusing lens.