Photon Transfer Curves (PTCs) at PhotonsToPhotos

Started 1 week ago | Discussions thread
JimKasson
JimKasson Forum Pro • Posts: 13,880
Re: Methodology

John Sheehy wrote:

J A C S wrote:

To make this more practical, it is true that doubling the ISO halves the actual highest photon count recorded in the RAW file? How about the intermediate ISOs?

That depends on how the cameras implement their ISO settings.

Depending on the analog gain used, a certain range of electron charges survives without clipping by the ADC. Depending on whether the firmware uses any further math after digitization, the range can get even smaller.
The range may be different for different color channels in some cameras, either through varied gain or varied integer scaling math.

Also, firmware often scales up digitized numbers and lowers the electron charge range to compensate for microlens losses at low f-numbers, to keep the "exposure triangle" intact, with fake exposure boost.

Correct. And I can detect many kinds of such trickery, and model much of it. Probably not all, though.

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