Lens elements shift due to heat or impact — focus and image plane separate. Zoom lenses most susceptible; calibration required.
Thermal expansion or mechanical displacement of lens elements—that's derailing, and it happens precisely when you least need it. The internal lens groups of a zoom lens lose their exact alignment when heat penetrates or when the lens takes a knock. Focus and the image plane decouple from each other—you focus, but the sharpness is no longer where you want it.
On set, you usually only notice it in the edit or, in the worst case, during review: depth of field becomes unpredictable, focus shifts appear sloppier than normal, and with critical settings, nothing lines up anymore. This problem is particularly common with zoom lenses because multiple lens groups change their position simultaneously. If even one of them shifts thermally or mechanically—bang—the internal optical bench calibration is ruined. You turn the focus ring, but the physical plane of focus doesn't follow linearly. It becomes particularly treacherous in extreme climatic conditions: extreme heat in the desert, rapid temperature changes when shooting indoors/outdoors, or moisture penetrating the mechanics.
Mechanical triggers are often brutal—a fall from a tripod, a camera dolly crash, or the lens hitting a cabinet door. Derailing also occurs due to minimal shifts that you don't see but are optically measurable. The solution: lenses must be sent to an optics workshop for calibration—this isn't just cleaning, but a readjustment of the lens elements. Therefore, some DoPs buy replacement zoom lenses for critical projects to avoid this dreaded failure.
Prevention measures include: shooting in extreme heat with thermal insulation (also for zoom lenses), regular calibration before shooting begins, and respectful handling. If you notice something is wrong—focus is off, or the zoom shows strange sharpness transitions—stop immediately and suspect the lens. Derailing can worsen over a series of shooting days and ruin multiple takes.