Biomicroscopic measurement of the optic disc with a high-power positive lens

To compare the magnification properties of four different indirect double aspheric fundus examination lenses for clinical disc biometry.

Experimental study in a model eye. The relationship between the true size of a fundus object and its image was calculated for each fundus lens for an ametropic range between -12.5 and +12.6 D using a slit lamp biomicroscope with adjustable beam length.

Equations for determining the correction factor p (degrees per millimeter) were calculated for each fundus lens. The factor can be used in calculations to determine true optic disc size. The total change in magnification of the system from myopia to hyperopia was -21.1% to +24.0% (60-D lens; Volk Opticals, Mentor, OH), -12.9% to +16.2% (Volk super 66 stereo fundus lens), -13.2% to +13.9% (Volk 78-D lens), and -13.3% to +14.0% (Volk super-field NC lens). When the fundus lens position was altered im relation to the model eye by +/-2 mm under myopic conditions, the change in magnification of the system was -4.3% to +5.7% (60-D lens), -4.6% to +6.1% (66 stereo fundus lens), -4.9% to +6.3% (78-D lens), and -5.9% to +7.8% (super-field NC lens). In the hyperopic condition the change was -2.7% to +3.6%, -3.4% to +4.5%, -3.6% to +4.8%, and -4.5% to +6.0%.

The study has shown that the use of a single magnification correction value for each fundus lens may not be appropriate. These findings have important implications for the way in which calculations for determining the true optic disc size and other structures of the posterior pole are performed using indirect biomicroscopy.