Depth distortion during slitlamp biomicroscopy of the fundus

The apparent depth of an optic nerve cup or apparent height of a fundus lesion changes with the optics used by an examiner. With slit-lamp biomicroscopy of the fundus, both the auxiliary fundus lens and the biomicroscope contribute to differences in magnification that produces distortion of the object. It is important to understand these differences so that inaccurate conclusions are not drawn and inappropriate therapy will not be instituted when the examiner uses different instruments for making serial observations or uses an unfamiliar instrument.