purpose. To investigate the magnification characteristic of the +90-D double-aspheric fundus examination lens for biomicroscopic measurement of the optic disc.

methods. A calibrated Gullstrand-type model eye adjusted for axial ametropia between −12.5 and +12.6 D was used to measure the change in magnification of the system with refractive error and variation in fundus lens position. A correction factor *p* (in degrees per millimeter) at different axial ametropias was also calculated.

results. The total change in magnification of the system from myopia to hyperopia was −15.1% to +13.7%. When the fundus lens position was altered with respect to the model eye by ±2 mm under myopic conditions, the change in magnification of the system was −4.8% to +8.1%. In the hyperopic condition the change was −4.8% to +6.0%. The fundus lens exhibited a linear relationship between *p* and the degree of ametropia of the model eye and a constant relationship between *p* and ametropia of −5 to +5 D.

conclusions. Axial ametropia has a significant effect on biomicroscopic measurement of the optic disc with the +90-D lens. Therefore, a correction factor (*p*) was calculated that can be used in calculations for determining true optic disc size. These findings may be important for improving clinical disc biometry.

^{ 1 }

^{ 2 }

^{ 3 }

^{ 4 }However, the aforementioned methods are not applicable in a routine clinical setting. Measurement of the optic nerve head is usually performed at the slit lamp biomicroscope using an auxiliary lens to overcome the high focal convergence of the examined eye.

*q*, in millimeters per degree)

^{ 1 }

^{ 5 }

^{ 6 }; the magnification of the instrumentation used to obtain the image (correction factor

*p*, in degrees per millimeter)

^{ 7 }; and the position of the instrumentation with respect to the eye. Thus, for calculating the true size of the optic disc, the total magnification of the system must be known.

^{ 8 }

*p*= (

*k*/17.453)(

*t*/

*s*), where

*k*is the ametropia of the eye + equivalent power of the eye,

*t*is the fundus object size (4 mm), and

*s*is its measured size on the slit lamp biomicroscope, we calculated the fundus lens correction factor

*p*.

^{ 7 }

^{ 9 }

*p*with variation in condensing lens position, measurements of the fundus object size were obtained as described when the fundus lens position was altered by ±2 mm relative to the model eye’s cornea under myopic and hyperopic conditions.

*p*. The value of

*p*is constant from −5 to +5 D. In the presence of a high refractive error, the fundus lens shows a linear relationship between correction factor

*p*and ametropia, which can be determined from linear regression analysis. The corresponding mean value of

*p*and the equation of the regression line to the results in Figure 4 are given in Table 1 . The regression line equation gives an estimate of the value of

*p*for any degree of ametropia.

*p*(in degrees per millimeter) when the lens position was altered by ±2 mm to the model eye’s cornea. The factor

*p*was 4.58 when the condensing lens was too close and 3.98 when the lens was too far away from the eye under myopic conditions. The change in factor

*p*was 4.71 when too close and 5.24 when too far away under hyperopic conditions.

^{ 1 }

^{ 5 }Littmann expressed the relationship between the size

*t*of an optic disc and the corresponding size

*s*of its image by the formula

*t*=

*pqs*in which the ocular factor

*q*is a variable specific to the examined eye. Several methods are available for determining

*q*for a human eye within ±20° of the optical axis, based on ametropia and keratometry,

^{ 1 }

^{ 5 }ametropia and axial length,

^{ 1 }

^{ 5 }and axial length only.

^{ 6 }

*p*refers to the instrumentation used to obtain the image. Because the factor

*p*remains unchanged for axial and refractive ametropias of the same degree,

^{ 10 }the Gullstrand-type model eye used for our study had a fixed corneal curvature and power of the intraocular lens, providing a model for ametropia only by varying the axial length of the model eye. Furthermore, the axial length is the most important factor for the change of the magnification due to ametropia.

^{ 6 }The change in magnification recorded in our setup is therefore most likely the maximum deviation to be expected in vivo, where a wide variation of the crystalline lens and the total axial length provide the refractive status of the patient’s eye.

*p*for the +90-D lens from Table 1 . In practice, it is important that the slit lamp and condensing lens be aligned correctly in front of the patient’s cornea and the optic disc be centered in the image field to maximize the repeatability of the experimental setup.

*p*and degree of ametropia of the model eye and a constant relationship between

*p*and ametropia of −5 to +5 D. As far as we are aware, this has not been reported previously for the +90-D lens. It means that the +90-D lens and the slit lamp biomicroscope build a pure telecentric device only for an ametropic range of −5 to +5 D. The reason for this lies in the difficulty of coinciding the focal plane of the condensing lens with the first principal plane of the eye in the presence of a high refractive error. It means that in a normal clinical setting at the slit lamp biomicroscope with the +90-D lens in the presence of a high refractive error, the image obtained may appear to be focused adequately to make a measurement with the slit beam, when in fact the focal plane of the condensing lens is not exactly at the first principal plane of the examined eye.

*p*of the 90-D lens may be useful in calculating the true optic disc size, without recourse to expensive technology.

^{ 1 }

^{ 2 }

^{ 3 }

^{ 4 }Furthermore, it makes optic disc measurements obtained by the +90-D lens comparable with biomicroscopic measurements obtained by other high-power positive lenses.

^{ 11 }This would be of particular importance when comparing the morphometric characteristics of the optic nerve head between individuals with regard to diagnosis and therapy.

**Figure 1.**

**Figure 1.**

**Figure 2.**

**Figure 2.**

**Figure 3.**

**Figure 3.**

**Figure 4.**

**Figure 4.**

Range of Ocular Refraction Investigated (D) | Factor p (deg/mm) | Range of Factor p (deg/mm) |
---|---|---|

−5 to +5 | 4.64 ± 0.02 (mean± SD) | 4.61–4.69 |

−12.5 to+12.6 | 0.025 A+ 4.66 | 4.30–5.00 |

**Figure 5.**

**Figure 5.**

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