… - I can and do both.
However, during actual scanning, exposure is just controlled by the LED-source, which is driven by 12bit DACs (an old schematic can be found here).
The camera is just too slow to respond to sudden changes in exposure time. The LED-source reacts immediately.
The different exposure settings of the LED-source were independently calibrated to have identical whitebalances. The reason is that the intensity-current curves of the red, green and blue LEDs used in the lightsource are different. So they react differently with changing current. Furthermore, the wavelength of a LED changes sligthly with varying temperature/driving current.
I am not sure that this technique of changing exposure via the lightsource would work well with pure white-light LEDs, as you would loose the possibility of aligning the color temperature for the different exposure settings/driving current. But I have not looked into that.
Only during experiments I do vary the exposure time of the camera. During scanning, the exposure time is fixed to something like 1/32-1/25 seconds. Small adjustments of analog gain and digital gain are used make sure that the internal sprocket area, where full intensity of the lightsource is visible, is imaged in the darkest exposure to values not larger than 240-250 (in the 8bpp MJPEG). Thus, the exposure is set by reference to the lightsource, not the film. This ensures that the full tonal range of the film can be captured.