Electronic Rolling Shutter

Most CMOS image sensors (to save one transistor per cell compared to a true "snapshot" shutter) use Electronic Rolling Shutter. Basically it implements two pointers to sensor pixels, both proceeding in the same line-scan order across the sensor,

One is erase pointer, the other one - readout. Erase pointer runs ahead discharging each photosensitive cell, then follows the readout one. Each pixel sees (and accumulates) the light for the same exposure time (from the moment erase pointer passes it till it is read out), but that happens at different time.

If you will make an image of a fast passing car with ERS with short exposure - it will all be sharp (no blurring), but the car will seem to lean backwards. The roof will be captured at earlier time than the wheels and this time difference across the frame can be as long as 1/12 of a second for the full frame of the MT9T001 3MPix sensor (it is equal to the frame readout time that is equal to 1 sec divided by the frame rate in most circumstances).

Some CMOS sensors have "global shutter" function mentioned in the specs but it is usually far from the real snapshot ones (as in the modern CCD-based camcorders). Such mode has simultaneous erase of all pixels, but the end of exposure is still determined by the it readout, so exposure time for each pixel will be different - pixels in the bottom of the frame will be exposed longer by the exposure time. Such mode is useful with the flash lamp, when it is triggered after all the pixels are erased but before the readout starts, but usually quite useless without such lamp (or external shutter of some kind - i.e. mechanical or LCD).