Thats a rather extreme example,...  & it seems unlikely.

That is: is it better for noise, range and color? Or does one make the
choice to keep two and drop one from the equation? Because if its better
for all three you would only need one ISO setting and not any
supplemental light sources.

You add flash/or lights as needed to make the Iso  100 image. Digital
does not solve the problems that exist beyond the scope of the camera-
lighting. & more likely It never will. Lighting is separate set of
issues and require knowledge. I can't seem to state this enough to
people, its something schools should teach :)

Is or is not the ISO "given", as you initially described the situation?

If by "given" you mean "fixed in advance", then:

If ISO is "given", "under exposure" (or "over exposure") describe the
situation correctly.

If the exposure is "given", then "under exposure" (etc) are also
appropriate because of historical use (even today ISO is not as
variable as exposure), but also because the assumptions are known.  But
if you don't like it, use "under amplified", "lack of gain" or
something and no one will quibble.

The thing is, you're the only person I have *ever* seen talk in terms of
"absolute exposure", or compare different ISOs at the same aperture and
shutter speed.  I'm not saying that's not a valid way to think about it,
I just don't see how it could be useful to me.  ISO 100 *does* give the
best quality, as long as you don't underexpose it, and it's a given that
a proper exposure at an elevated ISO rating is better than underexposing
at ISO 100.

Now, there is another angle to the whole thing, and that is the 12-bit
A/D conversion.  We've discussed previously how the dynamic range of
current sensors is not limited by the sensor's capability, but rather
by the A/D conversion.  This presents an interesting situation.  Imagine
that a camera used 16-bit A/D conversion.  Imagine that the extra range
actually *did* use all of the data available from the sensor.  You now
have a situation where higher ISO settings are meaningless, and the
camera would have to be marketed as (for example) ISO 100 with *no*
higher settings.  Imagine the outcry!

The simple fact that higher ISO settings exist and are useful tells
us that the A/D conversion is not using all of the data the sensor is
providing.  Higher ISOs are accomplished by amplifying the signal.
If you can usefully amplify the signal to ISO 800, that means there
was a signal there in the first place to amplify, one that *could*
have been used at ISO 100, but was ignored at that setting.  If no
data from the sensor were ignored, there would be nothing left to
amplify, and ISO 200 would just be ISO 100 with one stop less of
range and no actual advantage whatsoever.  That is, it *would* be
better to underexpose at ISO 100 and then push it in processing, to
avoid the amplification step.

So, it seems that either 16-bit A/D conversion is more complicated
to put into a camera than it sounds, or we are having our dynamic
range artificially limited in order to allow camera manufacturers
to say that their cameras can go to ISO 800 or whatever.

Of course, if the sensor can provide more range than a 16-bit
conversion would need, then there would still be room for higher
ISO settings.

[]

In signal processing we might use the term "headroom".  If the sound level
a system could capture was 8, and the loudest sound to be recorded was 4,
then we might say the headroom was 6dB (an engineering term for a factor
of 2 in linear voltage or current terms).  Similarly, if the maximum value
from your image sensor is 4095, and the white level of a particular image
were 2047, then you also have a factor of two headroom.  Were any part of
the image to exceed the maximum value - specular highlight for example -
we would say the value was clipped.

"Digitisation" is simply the process of converting analog to digital.

David

And most photographers will say: "So what?  Now let's meter the scene
and set the exposure."

In engineering terms, sure.  But few dedicated photographers are
engineers.  And most engineers who photograph as a hobby or interest do
so as a pastime, not as an extension of their engineering life.  Some
nerds excepted (not all engineers are nerds, far from it in my experience).

As to the 0 exposure adjustment, there is none, really.  None of the
sensors have an ISO 134.6767 (whatever) setting where they are unity
gain.  Few of the cameras record an ISO 100 (or other) sensitivity the
same as their competitors (or even models from the same co.)

You're raising a total non issue.  Or at best an issue that is
meaningful to you and very few others.  Photographers make photography
and think in photographic terms.

I know engineers who paint as well, but they don't talk about "pigment
carrying  linum usitatissimum  oils".  They talk about oil paints.

Cheers,
Alan

SNIP

The problem is that you essentially talk about Manual exposure
setting, which changes the ISO setting in an analog signal gain
control. However, in all other (non-manual) exposure settings, the
time x aperture metering settings change in function of the ISO
setting, in addition to the gain control. The latter applies to  the
majority of images shot, and thus ISO also changes the amount of
noise.

I'd suggest just using Exposure when using the EV, and EV + gain
control when shooting Manual.

Bart

Yes, when a photo-geek starts talking about "posterization", they are
referring to what anyone with a background in signal processing would
call "quantization noise".

http://en.wikipedia.org/wiki/Quantization_noise

Many fields have a maddening "terminological thicket" to penetrate...

I brought up the issue of quantization noise many months (over a year?)
ago on rpd.  It seemed to sail over the heads of just about everyone.  I
too did some signal processing work 10 or so years ago synthesizing
complex radar wave forms in realtime.  Quantization noise was not a
problem in the synthesis (16 bit DAC) but for the system under test with
a very high dyncamic range, it was a serious issue when the SNR was very
low.

For photography, the quantization noise is the noise we typically see at
high ISO settings in the shaddow areas of the image.  Some liken this
(erroneously) to film grain.  However film grain has dimension across
the image (x,y), as well as in color error (z),  whereas quantization
noise is dynamic (z) (color) only in digital cameras.

Cheers,
Alan.

In message <8PqJe.84115$G8.66978@text.news.blueyonder.co.uk>,
"David J Taylor"
<david-taylor@blueyonder.co.not-this-bit.nor-this-part.uk.invalid>
wrote:

I've understood "quantization" to be a specific kind of "posterization";
one where the values are integers or multiples of integers, whereas
posterization can include things like 1 2 4 5 7 8 10 11, etc.

"Quantization" would be clearer, I suppose, even if this is correct.  I
see the word posterized much more than I do quantized, nowadays, and
assumed the former was more in common usage.  People almost always refer
to an image with too few color levels to represent smooth gradients as
"posterized".

Welcome to usenet. :-)

You were quibbling over the word 'exposure.' The rest of the question
was about the dynamic range of the data resulting from the process of
exposure, not whether it *was* an exposure.

What you're asking is why we call a subset of the dynamic range falling
on the sensor an 'exposure.' And I answered it: Because it's exposed to
light.

You'd prefer there be an exposure, which is the sensor being shown the
light, and then another name for the process of amplification (per ISO),
conversion to digital, and optionally in-camera processing. Or perhaps a
name for each of those things. Well, there you go: Amplification, A/D
conversion, and processing.

What's the problem?

[]

You could use "dynamic range used" or "headroom".

- the image occupies 75% of the available dynamic range if the maximum
value is 3072 but the ADC maximum is 4095

- there is a headroom of 20% if the maximum signal is 800 and the ADC
limit is 1000

As dynamic range can also be used to describe the ratio of maximum to
minimum signal, I prefer the term "headroom".

I do now, however, see what you mean by exposure - in a histogram, that
part of the curve which is non-zero.  It's the occupied part of the total
system dynamic range.

David