Biomicroscope-Based Indirect Ophthalmoscopy: Key Factors Affecting Image Magnification

PURPOSE
To investigate factors which affect the linear magnification of the intermediate fundus image formed during indirect ophthalmoscopy conducted with a hand-held ophthalmoscopy lens and a slit-lamp biomicroscope.

METHODS
A conventional paraxial model, based on a ‘thin’ ophthalmoscopy lens and a ‘reduced’ eye, was used to develop a series of original equations to demonstrate the effects of the equivalent power (FE) and spherical ametropia (K) of the eye, and the equivalent power (FO) and position of the ophthalmoscopy lens (d) on linear image magnification. Our model’s predicted magnifications were compared with practical results found in an earlier published experimental study (Ansari-Shahrezaei et al., 2001), which used Volk ophthalmoscopy lenses in conjunction with physical model eyes with adjustable levels of axial ametropia.

RESULTS:
Our model’s magnification predictions, as a function of the eye's ametropia, were in good agreement with previous experimental measurements, provided that the equivalent powers of the Volk lenses were used, rather than their labelled ‘nominal’ powers. Magnification values typically changed by approximately ±10% over the practical range of each parameter when other parameters were held constant. In particular, normal variations in the equivalent power of the adult emmetropic eye resulted in magnifications which ranged from about 90-120% of the ‘nominal’ value given for an eye power of +60.00 D.

CONCLUSIONS:
All of the parameters considered in this study had marked effects on linear image magnification. The magnification values quoted by ophthalmoscopy lens manufacturers can be regarded as only approximations of those which may be found in clinical practice. Better estimates of magnification can be obtained by inserting the appropriate parameter values into the equations derived in this study, using, where appropriate, the equivalent power of the indirect ophthalmoscopy lens, rather than the lens’ labelled, ‘nominal’ power.

REFERENCES:
Ansari-Shahrezaei S, Maar N, Biowski R, Stur M. Biomicroscopic measurement of the optic disc with a high-power positive lens. Invest Ophthalmol Vis Sci 2001;42:153-7.

THREE intended learning objectives:
1) The ophthalmoscopy lens’ power provided by the manufacturer (e.g., Volk) may be a ‘nominal’ power, rather than the lens’ actual equivalent power.

2) The linear image magnification has an inversely proportional relationship to the lens’ equivalent power, as demonstrated by our main equation.

3) Indiscriminate use of a lens manufacturer’s image magnification values may lead to proportional errors of up to about 20% in the estimated dimensions of any retinal features measured using slit-lamp biomicroscope-based indirect ophthalmoscopy lenses.