Ophthalmoscope nun is a medical diagnosis device of the retinal disease by observing the retina of the human or animal. It comprises the objective lens, the relay lens and the eyepiece. Besides, the illuminating optical system is required because of the observation through small size of the pupil. Ophthalmoscope should secure a sufficient FOVffield of view) so that a wide area can be observed and should be arranged the focus mechanism because the human eyes are slightly different from each other.
Commonly, we use the way of compensation by moving the objective lens. This is the same principle with moving eyepiece in the microscope or telescope.