EXPOSURE ACCESS OF MECHANICAL CAMERAS
Very short exposure times associated with high speed recording require careful attention to timing of the event, illumination and recording. Exposure access indicates how closely a camera must be synchronized with light from the event. Cameras have either synchronous or continuous access.

CONTINUOUS ACCESS
Continuous access means that light from the subject will reach the film track whenever the capping shutter is open, because there is an optical path from the subject to the film track for all positions of the rotating mirror. (Rotating Drum Streak cameras which operate without a rotating mirror are also continuous access.) Most continuous access cameras achieve this with a film track which runs all or most of 360° around the mirror. Continuous access cameras often have a circular shape.

When recording with this type of camera, the capping shutter may be open for a relatively long period (e.g. 1/15 sec.) while the flash and event occur within a small portion of this period. The record is formed whenever light from the subject enters the camera. Less timing apparatus is then necessary, and the event needs to only to be illuminated and not necessarily triggered by the camera system. This type of camera is required when the event cannot be precisely initiated by a signal from the camera, such as when its time of occurrence is random or unpredictable.

SYNCHRONOUS
Synchronous access means that light will only reach the film track when the rotating mirror is in a specific range of positions. This requires the event be timed within a few degrees of the mirror rotation, so that the portion of interest coincides with mirror positions which send light to the film. In general, synchronous cameras are those of rectilinear or partial arc shape with film tracks that cover less than 360°.

The event and flash must be triggered by a synchronization signal. This signal is generated by the camera at an appropriate time before the mirror is in the right position. The advantage of synchronous access cameras is that they generally have larger effective apertures, offer much faster framing rates with larger frames and excellent resolution. Synchronous cameras are useful for those events which can be triggered on command by a signal from the camera.