On focusing the slit-lamp: Part II. "The fading-slit test"--verifying the ocular setting

The focusing-rod test is considered the gold standard for determining the correct setting of the slit-lamp ocular rings. This method is, however, somewhat time-consuming, considering the time it often takes to locate a rod every time one switches between slit-lamps or slit-lamp-mounted lasers. In this article we present an alternative test, which we call the fading-slit test, that requires only a second to perform, does not involve additional equipment (such as a rod), and can be applied at any time during the slit-lamp examination. However, while the focusing-rod test determines what setting should be used, the fading slit test can verify only whether the current setting is indeed the correct one and, therefore, it is not a complete substitute for the focusing-rod test. The fading-slit test is performed as follows: while focusing on any approximately flat surface (even the center of the patient's dome-shaped cornea would do) and using oblique illumination, gradually reduce the slit width until it disappears. Pay attention to whether the slit continuously thins, reaching a hairline width prior to its disappearance or, alternatively, whether, just before disappearance, the slit remains somewhat wide, gradually fading away, instead of thinning away. In the former case the ocular setting is correct; in the latter case it is incorrect.