In article , Eric Stevens wrote:

PeterN:
Because adaptation of methods of exposure varies, ISO should not
be considered an absolute measurement of sensitivity, but merely
a reasonable guideline. That is one of the reasons a lot of us
bracket exposures.

Sandman:
But the point is, ISO *was* an absolute measurement of
sensitivity, meaning that one film of ISO400 and another film of
ISO400 was equally sensitive to light, that was the entire point
of ISO.

Now, when digital rolled around, they adopted ISO to mean sensor
amplification, but the problem was that ISO is sensitivity over a
unit area, not over the exposed area (since that was pretty much
the same back in 135 days), but digital cameras can't use a
standard that is relevant to unit areas when each camera had
different amount of such unit areas

They can, when what matters is not the total amount of light falling
on the entire sensor but the amount of light which falls on a unit
area of sensor.

Incorrect on several accounts. First, the amount of light that falls on a
unit area is never "the matter" to the photographer. The end result is.

Secondly, when you have different amount of unit areas in play, whatever
"iso" one unit area may or may not be in relation to is irrelevant.

Thirdly, ISO for digital cameras does NOT means X amount of light gathered
per unit area, that's the *film* usage of ISO. "ISO" for digital cameras is
an arbitrary value to match the *lightness* of the film-equivalent of that
ISO setting.

So when you talk about "ISO" in digital cameras, it has absolutely nothing to
do with light per unit area other than the camera trying to emulate the
result of a ISO analog film. Which, depending on your sensors size means you
have to amplify it more or less than another camera.

Those pixels in the unit area don't care what is happening in the
neighbouring unit area. All that matters to them is the amount of
light which falls upon them.

They are also not more "sensitive" depending on your ISO settings. They are
fixed. With less total light, the signal needs to be amplified more to match
the brightness of the film ISO emulation step.



--
Sandman