Depth of field · Sensor size · Focus pulling
Move a control. The blur in the picture and the green band on the ruler follow real lens math.
At 50 mm on Super 35, get the actor sharp and the background lights fully melted (far limit under 3.5 m).
A teaching model. Distances: C-stand 1 m, actor 3 m, wall 12 m, measured from the camera. Circle of confusion assumed per sensor (FF 0.030, S35 0.025, APS-C 0.020, MFT 0.015, phone 0.008 mm). Crop factor is taken from sensor width for a 16:9 video frame, so it can differ slightly from diagonal figures. Thin-lens formulas; real lenses breathe and vary.