Sensitivity of the electronic subtraction method in evaluation of simulated optic disc changes

Electronic subtraction method can be used in evaluation of glaucomatous optic disc changes from sequential fundus photographs. To determine the resolving power of the subtraction method, minute changes in the diameter of the model optic cup of known dimensions were simulated by changing the magnification, i.e. the refraction, of an artificial test eye. The results show that with an overall magnification of 40 times a two-dimensional change of 25 micrometer or more in the optic disc can be detected with the electronic subtraction in optimal conditions. A larger magnification could improve the sensitivity, but at present it is restricted by unlinearity in the imaging system.