To explore the factors affecting the linear magnification of the intermedi- ate fundus image during indirect ophthalmoscopy with a slit-lamp biomicroscope.
A simple paraxial model, based on a ‘reduced’ eye and a ‘thin’ ophthal- moscopy lens, is used to develop equations showing the effects of the power and ametropia of the eye, and the equivalent power and position of the ophthalmos- copy lens on fundus magnification. Predicted magnifications are compared with practical results found in earlier published experimental studies, which used Volk ophthalmoscopy lenses in conjunction with physical model eyes with adjustable levels of axial ametropia.
The model's magnification predictions, as a function of the eye's ametro- pia, are in good agreement with previous experimental measurements, provided that the equivalent powers of the Volk lenses are used rather than their labelled nominal powers. Magnification values typically change by approximately ±10% over the practical range of each parameter if other parameters are held constant. In particular, normal variations in the equivalent power of the adult emmetropic eye result in magnifications which range from about 90–120% of the nominal value given for an eye power of +60.00 D. It is demonstrated that the recommended working distances for different powers of Volk ophthalmoscopy lenses approxi- mate optimal matching between the various pupils of the eye-Volk lens-slit-lamp biomicroscope system.
All the parameters considered have marked effects on magnifica- tion. The magnification values quoted by manufacturers can be regarded as only approximations of those which may be found in practice. Better estimates of mag- nification can be obtained by inserting the appropriate parameter values into the equations derived in this paper, using, where appropriate, the equivalent power of the indirect ophthalmoscopy lens, rather than the lens' labelled, ‘nominal’ power.
- Volk lenses
- equivalent power
- indirect ophthalmoscopy
- working distance