4.1.3 Exposure control
The F7 includes a global reset control that allows the charge stored in all photodiodes to be dumped and a line reset control that dumps only a selected line. The global reset is used primarily for still shot (SS) imaging of the type common in digital still cameras. In this mode, which uses an external mechanical shutter, the exposure cycle is very simple. To start the cycle, the sensor is globally reset, then, scanning is stopped, the external shutter is opened, an external flash may be fired and the external shutter is closed. After completion of the exposure, scanning is initiated. The external shutter may be a blade or electro-optic type, in which the entire sensor is exposed simultaneously, or a curtain shutter, in which a slot is drawn across the sensor, as is typical in 35mm film cameras. An equivalent cycle may be used where the shutter is replaced by a pulsed light source, but, in this situation, care must be taken to assure that the sensor is kept in the dark during scanning.
For flexibility, the F7 includes a separate counter to enable lines for reset, which be used together with the vertical readout counter to set a wide range of integration times between reset and readout of each line. This rolling shutter (RS) scan mode is an electronic equivalent of a curtain shutter. The counters can be configured to wrap around at the last line so that the integration time is constant for all lines. In the RS mode, the global reset is never activated.
Since the reset and readout functions are separately controlled, it is also possible to read the same line repeatedly without resetting it. This non-destructive reading permits monitoring of signal buildup during extended integration times to assure maximum dynamic range without overload. With careful signal management, high-speed binned monitoring can be followed by full-resolution readout without damage to the image data.
