ISO 100, OMD-EM1II and firmware 3.0?

Started 2 months ago | Questions thread
Chris Noble
Chris Noble Veteran Member • Posts: 3,144
Re: Basics of digital camera ISO

bobn2 wrote:

Rather than the provision of variable voltage gain, to produce the effect that Chris says is 'ISO' a much simpler expedient would be to simply reduce the ADC reference voltage (Fr information, almost invariably the 'fall scale' of an ADC is referred to an external or programmable reference voltage. There is a very good reason that cameras are not designed this way,

Actually, there are not one but several good reasons, for cameras and other mixed-signal circuits with multiple analog ranges. That is why your "much simpler expedient" approach is not feasible.

but as far as Chris' story goes, it would be just as good as the VGA, actually better because there would be none of the amplifier noise that he supposes the VGA is adding.

The VGA does not add appreciable noise. But (a) it amplifies the sensor noise and (b) if the sensor output range is not scaled to the A/D input range, A/D quantization noise rises, eventually above the input noise floor. Those are the reasons for the variable (actually fixed-step) VGA.

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