[Accommodation-convergence coupling using the stereomicroscope].

The term "instrument myopia" means an accommodative effort caused by the view into the microscope. This accommodation may be connected with the convergence position of the eye. Heterophoria should be developed if accommodation-induced convergence of the eyes does not harmonize with convergence of the ocular axes of a stereomicroscope. In 16 probands, an accommodation of 1.1 +/- 0.72 dpt and esophoria of +11.1 +/- 8.5 pdpt were measured at parallel ocular axes. The accommodation increased by 0.12 dpt/degree convergence with increasing convergence angle of the ocular axes. When the visual field was unrestricted, this line intersected the accommodation convergence line at 6.7 degrees. The esophoria decreased with increasing convergence angle and passed over to exophoria. At 8 degrees convergence, heterophoria amounted to -0.2 +/- 2.9 pdpt, and at 16 degrees exophoria was -12 +/- 9.5 pdpt. The minor degree of heterophoria and especially the very minor heterophoria at 8 degrees convergence support the view that the optimum convergence of the ocular axes is near where both accommodation convergence lines intersect.