Normally, a long focal length of 500 mm is also housed in a mount of 500 mm length. To achieve a handy, universally applicable, and compact solution, this lens employs a clever trick. The light path is simply folded using two high-precision ground ring mirrors. This results in a compact design and an aperture that is over 50% larger compared to a lens of the same focal length, as well as a very short minimum focusing distance of just 2 meters. Focusing is done manually. The lens can be used with either automatic exposure or manual exposure settings, depending on the camera. It is technically not possible to incorporate an iris diaphragm into the folded light path. If light reduction is necessary, it can be easily achieved with neutral density filters that screw onto the rear lens. There is a thread M34x0.5 available for this purpose. Practically, the lens does not have a fixed infinity stop. Focusing is done by swinging the focus point from out of focus to sharp and back to out of focus to find it securely. T2 connection, operating tip: The T2 adapter is screwed onto the optics, then the lens is locked in place. The side screws on the T2 adapter allow for adjustment after loosening, aligning the index upwards. Retightening the screws securely holds the optics and adapter in place. Advantages of a full-frame lens on a camera with an APS-C sensor: Save time in post-processing. Many lens errors such as distortion, corner softness, or vignetting occur in the outer areas of the image. When using a lens designed for full-frame on a camera with an APS-C sensor, these areas are not utilized. This results in maximum quality, eliminating the need for additional post-processing time to correct lens errors.