Why can a longer exposure blur motion?

A photographic exposure collects light over a time interval. If a subject’s image changes position during that interval, its light is recorded across a path rather than at one fixed position. This temporal accumulation can produce motion blur even when the subject would be in focus at a single instant.

The original diagram holds the camera fixed and gives a pointlike marker a constant projected motion rate. At 400 drawing units per second during a one-fifth-second exposure, its image travels eighty units. During one-hundredth of a second at that same rate, it travels four. These are distances traversed during exposure, not full visible streak widths for a finite-sized object.

The stationary background does not travel across the image in this model, so its edges can remain sharp while the moving marker leaves a trail. A faster-moving image can cover a longer path even if the exposure time is unchanged. Camera motion could add another source of blur, but it is excluded from this comparison.

Motion blur is therefore distinct from changing focus or cropping an image. Focus concerns how a point is formed optically, while cropping selects the image boundary. Exposure duration also affects collected light, but brightness depends on other conditions and is not computed by this drawing. The simple rate-times-time relationship explains motion spread under fixed-camera, constant-projected-velocity assumptions.