Aliasing
Definition
When fast motion looks wrong on camera — wagon wheels appearing to spin backwards in old movies is aliasing in action.
Technical definition
Undersampled frequencies fold back as f_alias = |f − n·f_s|. A 30 fps camera cannot distinguish wheel speeds differing by multiples of 30 Hz.
Example
Car wheels in films seem to spin backwards because the camera samples 24 times per second while the wheel advances almost — but not quite — one spoke per frame.
Common confusion
Aliasing is not a camera defect — it is mathematics. Any sampler (camera, microphone, sensor) aliases signals above half its rate unless filtered first.