Lens aberration — point highlights distort into ellipses under off-axis light. Shows on cheap zooms; premium glass minimizes it.
With side lighting—especially when the light source enters the lens obliquely from above or below—inexpensive zoom lenses can produce a characteristic optical defect: point highlights deform into ellipses instead of remaining perfect circles. This lateral ellipse is caused by asymmetrical aberrations in the lens group, which are no longer symmetrically corrected at extreme angles of incidence of light. The deformation follows the direction of the incident light—hence "lateral"—and is particularly disruptive when shiny surfaces, windows, or lights in the background degenerate into bars or butterfly shapes.
On set, you'll notice this immediately in backlight scenes or when the sun comes through a window at a shallow angle. A 24-70mm kit zoom will show this more clearly than a high-quality prime lens. The effect typically intensifies at the edges of the image field and decreases towards the center. Good standard zooms correct this through more complex lens elements—a real quality difference between budget and professional optics that is often overlooked in practice because it only occurs in specific lighting situations.
In post-production, a lateral ellipse cannot be reliably corrected without damaging the rest of the image. The best approach: pay attention to it on location. This means specifically avoiding extreme backlight positions with budget zooms, or positioning critical highlights more centrally in the frame where distortion is minimal. For productions with demanding lighting or high-key scenes, you should invest in better optics from the outset. Some DoPs consciously work with this defect for a visual "found footage" look—but that's the exception, not the solution. Optical quality begins with lens choice, not grading.