Shutter · Practical lights · Mains flicker
Class rule: start at 180°. Leave it only for a reason, and flicker is a reason.
Teaching model: lights pulse at 2× mains (|sin| waveform, approximate depth per type); the rolling shutter reads top to bottom in 26 ms (strip: solid window = top row, dashed = bottom row); brightness is held constant across angles; band contrast is drawn 4× stronger, the ± figures are the model values; real mains drift makes "still" bands creep.