Background: Ophthalmoscopy of the fundus in a patient with head tremor is facilitated by image stability, whereas such observation of a patient with nystagmus is made difficult by image movement. Why does this occur?
Methods: We offer an explanation of these observations and derive mathematical proofs for both direct and indirect ophthalmoscopes in various states of ametropia.
Results: By means of image displacement calculations, we have created a graphical display of refractive error versus image displacement, showing zero displacement in emmetropia in head tremor and exaggerated displacement during nystagmus, regardless of the refractive status of the eye.
Conclusions: Simple optics and a mathematical proof explain the clinical observations. The image stability in the patient with head tremor and the image instability in an eye with nystagmus are the consequences of the optical system of the eye.