ISO 100, OMD-EM1II and firmware 3.0?

Started 2 months ago | Questions thread
bobn2 Forum Pro • Posts: 61,730
Re: Calling your bluff

katastrofa wrote:

Can you explain why the noise decreased with ISO in the first pair of photos?

ADCs are noisy. The actual number of available bits is given by a parameter called 'ENOB' (effective number of bits) and is rarely the same as the full bit width of the ADC. For a fast 14 bit ADC it might be 11-12 bits. The bits you don't get are essentially full of noise. The role of the variable voltage gain is to boost the signal above this noise. So, lets compare the two images, both have the same exposure, but the 100 ISO one is using just the lower bits of the ADC, which are noisy. In the second, the sensor signal has had gain applied, which means it is using the upper bits of the ADC, which aren't noisy.

The D700 was the first camera to appear with Sony's column parallel 14-bit ADC (or actually the first one to make use of it, the Sonys at the time only use 12 bit conversion). The ADC has an ENOB of a bit over 13 bits (due to there being thousand operating in parallel, allowing them to work slowly - noise and speed are inextricably intertwined), it shows very little benefit from the gain, since the ADCs aren't noisy in the first place.

With respect to mFT, one of the advantages of its small pixels is that 12 bits is enough to completely (or nearly, with some modern sensors) encode the DR of the pixel output, so it actually has no need for 14 bit ADCs. The modern column parallel ADCs on both Sony and TPSCo FT sensors are something around 11 ENOB, so again the voltage gain doesn't do a lot.

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263, look deader.

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