Why is the Parfocal Distance of the Objective 60 millimeters?
Once the tube lens focal length was set to 200 millimeters, the parfocal distance of the objective has to be increased from the standard 45 millimeters. As explained in the section on tube length, the focal length of the objective is also increased in order to preserve the same magnification, and since 45 millimeters does not provide optimum space in this design, a high-quality image cannot be obtained. In practice, the CF N Plan Apo 60x oil with a mechanical tube length of 160 millimeters, believed to be the ultimate in finite objectives, is crowded with lenses in a limited space of 45 millimeters. When this finite system is replaced with an infinite system and the objective is divided into an objective and a tube lens, the focal length of the tube lens becomes the equivalent of approximately 150 millimeters. On this basis, we can calculate the parfocal distance to provide an optical performance which surpasses that of the finite system as follows: The finite system objective parfocal
