"Exposure" vs "Digitization

In message ,
John A. Stovall wrote:

On Sun, 07 Aug 2005 12:13:51 GMT, wrote:

After spending some time under the hood, so to speak, of RAW capture and
data, I find it increasingly difficult to use the term "exposure" to
refer to the relative degree of photon saturation in a JPEG or RAW at a
given ISO. The analog to slide film exposure is actually the analog
exposure on the sensor; the ISO settings of the digital camera are like
setting different ranges of exposure in a slide to be digitized by a
scanner.

Why then, do we call utilizing the specified range "exposure". I often
substitute the word "digitized" in this context, but it draws strange
reactions from some people.

I prefer the term "imaging".

Well, it's part of "imaging", but "imaging" is just too broad, IMO.
--


John P Sheehy

In message ,
(Paul Mitchum) wrote:

wrote:

The analog to slide film exposure is actually the analog exposure on the
sensor; the ISO settings of the digital camera are like setting different
ranges of exposure in a slide to be digitized by a scanner.

Why then, do we call utilizing the specified range "exposure".

Because when taking a picture, the sensor is exposed to light. When not
taking the picture, the sensor isn't exposed to light, particularly when
it comes to DSLRs.

Thanks for that worthless response. Did you even bother to try to
understand what my question was?

At any given ISO setting, only part of the range of sensor exposure is
converted to meaningful numbers. The sensor, I understand, receives an
exposure. How then do we call how well the smaller, digitized range is
utilized, "exposure". That was my pondering.
--


John P Sheehy

wrote:

In message ,
(Paul Mitchum) wrote:

wrote:

The analog to slide film exposure is actually the analog exposure on the
sensor; the ISO settings of the digital camera are like setting different
ranges of exposure in a slide to be digitized by a scanner.

Why then, do we call utilizing the specified range "exposure".

Because when taking a picture, the sensor is exposed to light. When not
taking the picture, the sensor isn't exposed to light, particularly when
it comes to DSLRs.

Thanks for that worthless response.

Welcome to usenet. :-)

Did you even bother to try to understand what my question was?

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.

At any given ISO setting, only part of the range of sensor exposure is
converted to meaningful numbers. The sensor, I understand, receives an
exposure. How then do we call how well the smaller, digitized range is
utilized, "exposure". That was my pondering.

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?

Paul Mitchum wrote:



Did you even bother to try to understand what my question was?


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.

Paul, I admire your restraint in answering the questions above.


What's the problem?

None at all.

Cheers,
Alan


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David J Taylor wrote:

Alan Browne wrote:
[]

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.


Thanks, Alan.

I would be surprised if quantization noise were an issue at low SNR (i.e.
high ISO settings) in a digital camera, though, as the signal is amplified
before the ADC, so that the photon noise should swamp the quantisation
noise. However, I haven't sat down and done the sums....

A good point, however a closeup examination of photographs at 1600 and
3200 show discrete pixel jumps that are similar in nature to the effects
of quantization noise. It is (as you say) disguised by other things in
the chain (ADC, unknown local pre-ADC amps), but it does indeed bear
resemblance to quantization noise.

Cheers,
Alan.
--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.

wrote:
[]
At any given ISO setting, only part of the range of sensor exposure is
converted to meaningful numbers. The sensor, I understand, receives
an exposure. How then do we call how well the smaller, digitized
range is utilized, "exposure". That was my pondering.

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

Alan Browne wrote:
[]
A good point, however a closeup examination of photographs at 1600 and
3200 show discrete pixel jumps that are similar in nature to the
effects of quantization noise. It is (as you say) disguised by other
things in the chain (ADC, unknown local pre-ADC amps), but it does
indeed bear resemblance to quantization noise.

Cheers,
Alan.

To get the higher ISOs, for example, and using simple numbers:

- in normal mode, the converter digitises a voltage range of 0..4V to a
digital range of 0..4000. This means it must be accurate to 0.001V. The
ADC will work by comparing a signal with a 4V reference value.

- in ISO 3200 mode, the signal range is now just 0..1V. To do this, you
could

either:

-- quadruple the values from the ADC, turning it into a device digitising
0..1V to a digital range of 0..1000, but multiplying each result by 4, so
that 0..1V returns digital values of 0..4000. The quantisation steps are
still 0.001V, but as the digital values are quadrupled, the digital levels
in the image will be in steps of 4; 0, 4, 8, 12, etc.

or:

-- reduce the reference voltage in the converter so that it measures the
analog voltage against a 1V reference, but still returns values 0..4000.
The quantisation steps are now 0.0025V. Whilst the analog accuracy of the
converter may not justify the full 0,0025V steps, digitising this way may
produce a slightly more accurate result than simply quadrupling the
values. The digital levels will still be in steps of 1.

Perhaps there is a simple "double the output of the converter" happening
in cameras where this quantisation is observed? Perhaps both methods are
used to get two extra ISO steps?

Cheers,
David

David J Taylor wrote:
Perhaps there is a simple "double the output of the converter"
happening in cameras where this quantisation is observed? Perhaps
both methods are used to get two extra ISO steps?

Yuk, I hope not.

It would be possible to test this but I'd rather take photos :-)

-Mike

Mike Warren wrote:
David J Taylor wrote:
Perhaps there is a simple "double the output of the converter"
happening in cameras where this quantisation is observed? Perhaps
both methods are used to get two extra ISO steps?

Yuk, I hope not.

It would be possible to test this but I'd rather take photos :-)

-Mike

It may be that the realistic maximum speed is ISO 800, but marketing
demands ISO 1600 and ISO 3200. Nothing extra to be extracted from the
sensor, so just double the ADC output and perhaps dither it a little to
disguise the action?

David

In message ,
Alan Browne wrote:

A good point, however a closeup examination of photographs at 1600 and
3200 show discrete pixel jumps that are similar in nature to the effects
of quantization noise. It is (as you say) disguised by other things in
the chain (ADC, unknown local pre-ADC amps), but it does indeed bear
resemblance to quantization noise.

If you want to see noise that is mainly quantization from the
digitization, just crank up the shadows of an under-exposed ISO 100
image. The high-ISO shadows are a combination of that and sensor noise
with a tad of amplifier noise.
--


John P Sheehy