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The Fundus Image
The condensing lens, held at its focal length, produces a real, but inverted, aerial imageof the fundus. The retina is illuminated by a light source in the headset of the instrument.
A stereoscopic image is made possible by prisms within the instrument, which effectively reduce the examiner's interpupillary distance, allowing the pupils of the examiner to be imaged within the patient's dilated pupil.
Since a real image is formed between the condensing lens and the examiner, viewing is facilitated by the placement of plus lenses in the oculars of the instrument. This reduces the amount of accommodation necessary to see the image.
The fundus image is inverted (i.e., it is upside down and reversed right for left). However, only the image is inverted, not the location of the fundus. In other words, you see that part of the fundus where you are looking. If you look into the superior nasal fundus, your image is not of the inferior temporal fundus. What you see in the condensing lens when you look into the superior nasal fundus is an inverted view of the superior nasal fundus.
At any particular clock-hour position, you see fundus as if you were standing on the ora serrata looking back toward the posterior fundus.
When the patient's eye is held still, movement by the examiner in one direction produces a movement of the fundus image in the opposite direction. When the examiner moves from right to left, the fundus image moves from left to right.
The examiner's head moves toward a fundus finding to move it more to the center in the condensing lens. To view a portion of the fundus that is imaged peripherally in the lens, the examiner physically moves toward the periphery of the lens.
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