Frame Error - Frame Edge Shading
"Frame Edge Shading" is the unintentional darkening of one or more edges of a frame.
Frame edge shading negatively impacts the customer experience by obscuring in-frame content, or revealing visual cue to the filmmaking process, removing the suspension of disbelief. Sometimes creative intent to create that "film-like" experience.
Moving Camera with visible vignetting with matte box or other obstructing objects.
How to Prevent:
Check edges while shooting and adjust any incorrectly oriented lens hoods.
Using a radial graduated neutral density (GND) filter can help when the source of darkening is natural vignetting.
Review and confirm if this is creative intent, otherwise, scale the image or introduce VFX to conceal.