On focusing the slit-lamp: Part I. An inaccurate ocular setting--what is there to lose?

Adjustments of the diopter rings on the slit-lamp oculars compensate for three independent factors: the observer's refractive error, the observer's state of accommodation, and any instrument misalignment. An inaccurate setting of these rings will significantly affect the slit-lamp image quality, as well as the precision and accuracy of slit-lamp mounted lasers, such as are used for retinal photocoagulation, YAG capsulotomy, and laser trabeculoplasty. For each slit-lamp and microscope-related examination technique, photographic technique, and laser procedure, the consequences of an incorrect ocular setting are discussed. Data are presented on the preferred ocular settings of several random observers, highlighting both interobserver and intraobserver variability. Finally, photographs demonstrate the image deterioration that occurs when an incorrect ocular setting is used. It is shown that for younger observers, one's distance refraction is only seldom the correct setting of the slit-lamp oculars, and that on different instruments, one should not be surprised to find somewhat different settings. Examiners are urged to check the accuracy of their habitual setting, if only to find out whether the improvement in image quality is worth the extra bother.