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

methods. 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.

results. 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%.

conclusions. 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.

^{ 1 }first examined the fundus with a plano-convex lens of approximately +60 D using the slit lamp biomicroscope, but the technique was not widely accepted because of aberration and difficulty of use. With the introduction of the double aspheric 60-D lens in 1982 (Volk Opticals, Mentor, OH), the technique started to gain popularity for routine stereoscopic examination of the posterior pole.

^{ 2 }

^{ 3 }

^{ 4 }

^{ 5 }

^{ 6 }

^{ 7 }

^{ 8 }

*q*; millimeters per degree),

^{ 9 }

^{ 10 }

^{ 11 }

^{ 12 }magnification due to the condensing lens used to obtain the image (correction factor

*p*; degrees per millimeter)

^{ 13 }and the position of the condensing lens with respect to the eye

^{ 8 }; thus, for calculating the absolute dimensions of this image the total magnification of the system must be known.

^{ 14 }

*p*was then determined according to the formula:

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

*t*is the fundus object size (the diameter), and

*s*is the measured size of the fundus object’s diameter on the slit lamp biomicroscope.

*p*for each fundus lens. With the 60-D lens, the fundus lens correction factor is constant over an acceptable range of ametropia (−12.5 to +12.6 D). The value of

*p*for the 66 stereo fundus lens, 78-D lens, and super-field NC lens is constant from −5 to +5 D. In the presence of high refractive error, these fundus lenses show a linear relationship between correction factor

*p*and ametropia, which can be determined from linear regression analysis. The reason for this, compared with the 60-D lens, lies in the difficulty of making the focal plane of the condensing lens coincide 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 these fundus lenses (66-D, 78-D, and super-field NC lenses), in 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*for each fundus lens are given in Table 1 . Ophthalmologists can determine the

*p*for the 66 stereo fundus lens, 78-D lens, and super-field NC lens for an ametropic range over ±5 D from regression line equations as shown in Table 1 . These regression line equations give an estimate of the fundus lens correction factor

*p*for any degree of ametropia. The 95% confidence intervals for repeated measurements of

*s*are also shown in Table 1 . These are expressed as percentages of

*s*and are within acceptable limits.

*p*for these fundus lenses increases from myopia to hyperopia (see Fig. 2 ).

^{ 15 }Therefore, for clinical purposes, it is usually sufficient to know whether the optic disc is abnormally large, medium, or abnormally small.

*p*(see Table 1 ), ophthalmologists can determine the true size of the optic disc (

*t*) according to Littmann’s formula:

*s*is the optic disc diameter measured at the slit lamp biomicroscope, which must be calibrated before the measurement, and factor

*q*is the relationship of the real diameter of the optic disc measured in millimeters to the angular diameter, with which the optic disc is reflected through the optical system of the eye in its exterior space. It is a variable dependent on the optical dimensions of the patient’s eye and not the fundus imaging system. Several methods are available for determining the ocular factor (

*q*in millimeters per degree) for a human eye based on ametropia and keratometry,

^{ 9 }

^{ 10 }

^{ 11 }ametropia and axial length,

^{ 9 }

^{ 10 }

^{ 11 }and axial length only.

^{ 12 }

*m*is the total linear magnification of the system. It can be determined according to the formula:

*F*

_{e}is the equivalent power of the eye,

*A*the ametropia of the eye,

*F*

_{c}the power of the condensing lens,

*e*the position of the first principal plane of the eye, and

*d*the working distance of the condensing lens. The minus sign indicates an inverted image. It is not possible to collect this amount of data in clinical practice, and thus the true optic disc size has to be estimated on the basis of fewer, easily obtained variables, as has been described.

^{ 17 }This is especially important in the identification of optic discs with glaucomatous optic neuropathy.

^{ 18 }

^{ 19 }

^{ 2 }

^{ 3 }

^{ 20 }

^{ 21 }

^{ 4 }

^{ 5 }

^{ 6 }by comparing the optic disc diameter measured at the slit lamp biomicroscope and its true size obtained by photogrammetry or Heidelberg Retina Tomograph, (Heidelberg Engineering, Heidelberg, Germany) to assess a conversion factor or equation for each fundus lens. A single correction factor or equation can provide only a rough estimation of the optic disc size (see Fig. 3 ).

^{ 16 }Spencer and Vernon

^{ 2 }showed that according to a single conversion factor, the Volk 78-D lens gives larger measurements of the optic disc than photogrammetry (using ametropia and keratometry)

^{ 22 }by 0.41 mm.

^{ 4 }

^{ 5 }

^{ 6 }However, in the analysis of measurement method comparison data, neither the correlation coefficient nor techniques such as regression analysis are appropriate.

^{ 14 }

^{ 23 }The correlation coefficient measures the strength of a relation between two variables, not the agreement between them. Perfect agreement is attained only if the points of measurements lie along the line of equality, but perfect correlation is attained if the points of measurements lie along any straight line.

^{ 23 }

*p*(see Table 1 ) allows slit lamp biomicroscopic measurement of the real optic disc size for any degree of ametropia and makes biomicroscopic measurements with different fundus lenses comparable. Furthermore, the factor

*p*can be used in the measurement of other structures of the posterior pole, such as retinal or choroidal tumors, and in the assessment of macular degeneration, by measuring the extent of the disease and its distance from the center of the foveal avascular zone. This would be of particular importance when comparing the morphometric characteristics of a fundus landmark of interest between individuals with regard to diagnosis and therapy. Therefore fundus landmarks should be measured in absolute size units (millimeters), instead of using the interindividually variable disc diameter as a measurement unit. It should be considered, however, that the ocular correction factor

*q*can be used with sufficient accuracy within ±20° of the optical axis.

^{ 9 }

^{ 12 }

**Figure 1.**

**Figure 1.**

**Figure 2.**

**Figure 2.**

Fundus Lens | Factor p (deg/mm) | Range of Factor p (deg/mm) | 95% Confidence Interval for Repeated Measurements of s (%) | Range of Ocular Refraction Investigated (D) |
---|---|---|---|---|

Volk 60-D lens | 3.04± 0.04 (mean± SD) | 2.96–3.08 | −12.5 to +12.6 | |

0.001 A+ 3.03 | 2.96–3.08 | +3.24 to−3.67 | −12.5 to+12.6 | |

Volk 66 stereo fundus lens | 3.56± 0.06 (mean± SD) | 3.46–3.63 | −5 to+5 | |

0.020 A + 3.54 | 3.26–3.75 | +3.89 to−4.25 | −12.5 to+12.6 | |

Volk 78-D lens | 3.69± 0.06 (mean± SD) | 3.61–3.78 | −5 to+5 | |

0.017 A+ 3.65 | 3.36–3.93 | +3.97 to −4.38 | −12.5 to+12.6 | |

Volk super-field NC lens | 4.61± 0.03 (mean± SD) | 4.57–4.66 | −5 to+5 | |

0.025 A+ 4.58 | 4.22–4.92 | +4.50 to −4.93 | −12.5 to+12.6 |

**Figure 3.**

**Figure 3.**

**Figure 4.**

**Figure 4.**

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