ISO 100, OMD-EM1II and firmware 3.0?

Started 2 months ago | Questions thread
bobn2
bobn2 Forum Pro • Posts: 61,699
Re: Basics of digital camera ISO

Chris Noble wrote:

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.

It's feasible, just not very sensible, because lowering the reference voltage whilst it will =multiply the digital numbers produced by the same input voltage, won;'t to anything with respect to what the voltage gain is actually for, which is boosting teh sensor signal up above the ADC noise.

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.

Sorry, that still isn't right. The VGA amplifies the sensor signal and noise equally, since the  noise is not something separate, it is just a variation on the signal. So the VGA makes no difference to the sensor SNR. What it does is boost the signal level up above the (fixed) ADC noise floor. Sensors with column ADCs tend to be much more ISOless, because the thousands of ADCs can operate much slower, so can be much quieter. Thus there is less need for variable gain. A/D quantisation noise is not the culprit, because the ADC has a big enough enob (there's a phrase to conjure with) to completely encode the DR given by the sensor at those exposure levels.

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263, look deader.

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