Why Use a 25 millimeters Objective Thread Size?
When the focal length of the tube lens is increased, the focal length of the objective must also increase. There is a limit to the objective pupil diameter (effective diameter remaining after the limits of the objective thread size), so a high numerical aperture (N.A.) cannot be obtained. Thus, the N.A. of low-power lenses is critically affected. At present, other manufacturers use a 20.32 millimeters thread size, but as mentioned above, Nikon uses 25 millimeters and is able to attain high numerical apertures.
