A camera with a short exposure time and good human reflex are required to image a tennis ball in flight. The awesome rapidity of microscopic motion would overtax the shutter of a camera and the human reflex. Therefore, it has to be set going by a “clocking” flash of light (pump pulse) before it can be captured by a precisely timed “probe” flash. Timing synchronism between the pump (initiating) and probe (capturing) pulse obviates the need for human reflex and the shutter of the camera is replaced by the ultrashort flash of probing light. The observation speed is improved in proportion to the shortness of the light flashes used for initiating and recording the motion (movie). This pump-probe approach – when implemented with femtosecond-duration flashes of laser light – opened the door to real-time observation of chemical reactions at the end of the past millennium (1999 Nobel Prize in Chemistry).

Electrons move a thousand times faster than atomic nuclei, hence recording their motion requires thousand times shorter, i.e. attosecond-scale, shutter time. Just as the amplitude variation (dashed line) of a femtosecond pulse triggers and probes nuclear motion in Zewail’s femtosecond photography Zewail’s femtosecond photography, an attosecond ultraviolet or X-ray pulse coming in synchrony with a few-cycle laser pulse of controlled waveform pulse may be used for starting or capturing electronic motion in real time. The role of the attosecond “starter gun” or “shutter” can alternatively be played by the central half cycle of the controlled electric field of a few-cycle wave of visible light. In the first realisation of attosecond "photography" an attosecond ultraviolet pulse sets atomic electrons free and the central half wave cycle of a waveform-controlled few-cycle laser pulse recorded the history of their emission.