Fig. 2. A 5.5 D lens is very good for magnifying lesions of the horse fundus.
that of the horse’s iris and pupil. The light is di- rected into the horse’s eye so that the tapetal reflec- tion is obtained and the lens is moved to and fro until a sharp clear image is produced. The observ- er-patient distance is approximately 50–75 cm. In horses a 20 diopter (D) condensing lens is the
most versatile in use, although the image is mini- fied. A 20 D lens minifies the fundic view with 0.79 and 0.84 magnification laterally and axi- ally, respectively. The 20 D lens provides a nice panoramic, screening view of the equine fundus, but it is not satisfactory for detailed highly magnified observations. Indirect ophthalmoscopy with a 14 D lens provides a magnified view of 1.18lateral mag- nification and 1.86 axial magnification. Indirect ophthalmoscopy with a 5.5 D lens provides 3.88 lateral magnification and 20.10 axial magnifica- tion in the horse.
Direct Ophthalmoscopy
The use of a standard direct ophthalmoscope pro- duces an upright image of greater magnification than is possible with the indirect ophthalmoscope when used close to the patient’s eye (Fig. 5). How- ever, viewing the fundus directly along a beam of light necessarily restricts the field of view. The direct ophthalmoscope provides the most magnified view of the fundus in the horse, with a lateral mag-
Fig. 4. This is the image of a horse fundus observed with the indirect ophthalmoscopy technique.
nification of 7.9 and an axial magnification of 8.4. Both distant direct ophthalmoscopy and close direct ophthalmoscopy should form part of a direct ophthalmoscopic examination.
Fig. 5. The direct ophthalmoscope is the basic instrument used to view the horse retina and optic disc.
Fig. 3. The image with the indirect ophthalmoscope is upside down and reversed.
8 2014 Vol. 60 AAEP PROCEEDINGS
Fig. 6. The direct ophthalmoscope has a large aperture for large pupils.
