D3X vs 5D2 - myth and reality

Started Jan 26, 2009 | Discussions thread
OP GaborSch Veteran Member • Posts: 7,203
About ISO settings

Tom Christiansen wrote:

I'm not sure what you mean when you say that some of the ISOs
are fake. I'd thought all but one of them was! Am I mistaken,
or are we using the same terminology, differently?

There are several concepts of ISO; even a "push" in raw processing can be interpreted as "proper ISO". Let's stick to technically relevant issues:

1. the sensor's sensitivity is fixed, it depends on the quantum effifiency, on microlenses and on the pigment in the color filters (and on the angle of the infalling light, which is NOT fixed...) We can't do anything about this aspect.

2. different "results" from the same number of captured and converted photons can be achieved by applying analog gain before the A/D conversion. This step does have a huge impact on the noise, which can not me made up for later.

This is, what I (and I think many others) understand as "true ISO" or "ISO associated with analog gain".

3. Digital manipulations can change the final outcome, like "pushing the exposure" and "noise reduction", however their effect is on a different level.

Many cameras offer built-in options to control the necessary adjustments, in forms of ISO settings, which do not affect the data in the sense of #2. There are several variations of this adjustment. Most MFDBs don't do anything but record the setting as metadata with the raw data. Unfortunately, manufacturers of DSLRs have the dumb habit of manipulating the data in-camera. This is ok for JPEG, but horrendeous for the raw data.

This is, what I call "fake ISO": it has the the same effect as adjusting the "exposure" in raw processing, except

1. it reduces the dynamic range if the adjustment is "stretching",

2. it reduces the number of pixel levens if the adjustment is "shrinking" (this is the lesser issue).

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