Applied Scientific Instrumentation Inc.’s CRIFF (continuous reflective-interface feedback focus) system reduces focus drift in high-power microscopy by monitoring the distance between the objective lens and the specimen’s coverslip. A near-IR laser-based optical detector senses minute changes in the distance and provides a feedback signal to the Z-drive focus controller. The system compensates for focus changes caused by temperature or mechanical variations, maintaining a specimen’s focus for hours with <100 nm of drift. The module fits on the camera port of most microscopes. It can be used with total internal reflection fluorescence objectives (NA 1.45) and ordinary coverslips, or with lower-NA objectives and reflective-coated coverslips.