ISO 100, OMD-EM1II and firmware 3.0?

Started 2 months ago | Questions thread
Chris Noble
Chris Noble Veteran Member • Posts: 3,154
Re: Basics of digital camera ISO

Architeuthis wrote:

Hi Chris,

I thank you and the others for the valuable information.

I have just one questio at the time:

Chris Noble wrote:

The sensor, analog gain (ISO), A/D, and digital gain (in that signal order) are distinct.

Can one summarize this statement as follows?

#1. Sensor has always same amount of photoelectrons in a given pixel after identical exposure. (Sometimes the readout current can be at different size, when the sensor can be operated at two different supply voltages as e.g. is the case with the dual gain in the GH5s ???)

I don't know about variable sensor bias. Sounds reasonable.

#2.: different native ISO settings are produced by different analog amplifications of the signal (Outcome is likely a stable voltage after current to voltage conversion with different gain setting of the I/V converter). Base ISO is when the I/V converter is operated at its lowest gain possible (or reasonable).

Yes. And you know about I/V! Good. "Lowest reasonable" is when a full pixel well generates a signal that is at the full scale of the A/D input.

#3.: A/D converter has always the same range of input sensitivity (e.g. 0V-1V (according to bobn2 it is more likely around 1V than my intitial 5V guess))

The power rails keep on going down. I think between 1.5V and 2.5V is about right. You want the input noise to be below 1 LSB.

#4.: extended ISO results from postprocessing the digital numbers after A/D conversion. Identical results can be obtained when processinmg the raw file in e.g. LR.

There is often some "light" digital processing (i.e. in the camera) at every ISO step in addition to the Bayer de-convolution. But "extended" or "interpolated" ISO settings refer to digital gain to supplement the analog gain. ("Postprocessing" is on the computer after the Raw file has been recorded, "processing" is in the camera before the Raw file is recorded). By the way "gain" can be unity, >1 or <1.


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