Diagnosis
The focus ring on your UHD50X is mechanically loose because the internal helical focus mechanism has likely worn out or its calibration has slipped. This is a known hardware issue in some units, often accelerated by frequent adjustments, thermal expansion/contraction from projector heat cycles, or physical stress from mounting angles.
How to Fix It
Since the focus assembly is a sealed optical component, there is no safe user-serviceable repair. You must initiate a warranty or paid repair through Optoma Support. Have your projector's serial number ready. If under warranty, they will typically provide an RMA for a service center to repair or replace the lens/focus block. While waiting, document the issue with a video showing the ring's excessive play.
If That Didn't Work
If a warranty claim is denied or repair is delayed, you can attempt a temporary stabilization. Once you achieve a sharp image, carefully wedge a small, non-abrasive material like a folded paper shim or a piece of soft foam between the focus ring and the projector housing to inhibit movement. More permanently, avoid using any lens shift or zoom after focusing, as engaging these can torque the lens barrel and worsen the drift.
Heads Up
Forcing or repeatedly adjusting a loose focus ring can grind internal components, leading to permanent lens element misalignment, debris inside the optics, or complete focus mechanism failure. Do not attempt to open the lens assembly yourself, as this voids warranties and exposes delicate components to dust.