How does the ophthalmoscope work?
JUN 18, 2013
I have a question about the ophthalmoscope. To examine myopic patients you use the negative red numbers and to examine hyperopic patients you use the green positive numbers. I want to know how a doctor with an ametropic eye observes a patient. If you're farsighted looking into the eye of a nearsighted or myopic patient, you need to concave the lens but this will make the doctor's vision worse. How do you balance the two?
This is not the kind of a question a typical patient asks and I appreciate your interest. The ophthalmoscope was invented by Helmholtz in 1850. For a century it was the only way to view the fundus of the eye. Most ophthalmologists have deserted the direct ophthalmoscope for a different device, an indirect ophthalmoscope, which provides a better view of the peripheral retina and assists in the discovery of retinal holes, etc.
The direct ophthalmoscope has converging and diverging lenses which can be rotated by the ophthalmologist to find the best possible focus and view of the retina of the patient. If the doctor and the patient both see clearly without glasses, the doctor will likely use a Plano (no power) lens to focus the device. Since effective lens power is additive (algebraic), the refractive error of the doctor and the refractive error of the patient can be compensated for by rotating the lens wheel of the ophthalmoscope. Sometimes this is done by trial and error such that the doctor starts at zero and begins turning the wheel to clear the view and turns it back the opposite way if the image is getting worse, rather than better. At other times, the doctor will rapidly and roughly calculate the algebraic difference between his/her glasses correction number and that of the patient.