Tube Lens Focal Length
As the focal length of the tube lens is increased, the distance to the intermediate image plane also increases, which results in a longer overall tube length. Tube lengths between 200 and 250 millimeters are considered optimal, because longer focal lengths will produce a smaller off-axis angle for diagonal light rays, reducing system artifacts. Longer tube lengths also increase the flexibility of the system with regard to the design of accessory components.
The advantages of a longer tube lens focal length becomes apparent when comparing systems having a focal length range between 160 and 250 millimeters. When the tutorial initializes, the Focal Length slider is set to a value of 200 millimeters and the objective exit Pupil Diameter equals 17 millimeters. The Focal Length slider is utilized to adjust the tube lens focal length to a new value, which either increases or decreases the Infinity Space between the objective and the tube lens. In addition, moving this slider to longer tube lengths decreases the angle θ between on-axis and off-axis light rays passing through the system. At constant focal length, variations in the Pupil Diameter slider produce much larger changes in θ, which affects the image quality seen in the microscope.
Reduction in the off-axis diagonal wave flux angle can approach a significant percentage with the longer focal length optical system. The reduced angle of the oblique light rays produces correspondingly smaller shifts in both on-axis and off-axis rays passing through accessory components (DIC prisms, phase rings, dichroic mirrors, etc.), which improves the efficiency of the microscope. Dramatic enhancement in contrast levels observed with epi-fluorescence illuminators in infinity-corrected systems is attributed to the optical advantage of longer tube lens focal lengths.