Factors affecting image magnification in indirect ophthalmoscopy with Volk or similar lenses and a biomicroscope.

PURPOSE: To explore the factors affecting the linear magnification of the intermediate fundus image during indirect ophthalmoscopy with a slit-lamp biomicroscope. METHODS: A simple paraxial model, based on a 'reduced' eye and a 'thin' ophthalmoscopy 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 ophthalmoscopy 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. RESULTS: The model's magnification predictions, as a function of the eye's ametropia, 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 approximate optimal matching between the various pupils of the eye-Volk lens-slit-lamp biomicroscope system. CONCLUSIONS: All the parameters considered have marked effects on magnification. The magnification values quoted by manufacturers can be regarded as only approximations of those which may be found in practice. Better estimates of magnification 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.

Customised aberration-controlling corrections for keratoconic patients using contact lenses.

Technological advancements in the design of soft and scleral contact lenses have led to the development of customised, aberration-controlling corrections for patients with keratoconus. As the number of contact lens manufacturers producing wavefront-guided corrections continues to expand, clinical interest in this customisable technology is also increasing among both patients and practitioners. This review outlines key issues surrounding the measurement of ocular aberrations for patients with keratoconus, with a particular focus on the possible factors affecting the repeatability of Hartmann-Shack aberrometry measurements. This review also discusses and compares the relative successes of studies investigating the design and fitting of soft and scleral customised contact lenses for patients with keratoconus. A series of key limitations that should be considered before designing customised contact lens corrections is also described. Despite the challenges of producing and fitting customised lenses, improvements in visual performance and comfortable wearing times, as provided by these lenses, could help to reduce the rate of keratoplasty in keratoconic patients, thereby significantly reducing clinical issues related to corneal graft surgery. Furthermore, enhancements in optical correction, provided by customised lenses, could lead to increased independence, particularly among young adult keratoconic patients, therefore leading to improvements in quality of life.