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Of late I've seen a couple discussions of the importance of not underexposing a
digital image. Specifically this is because if you have 6 stops of dynamic
range, half the data, or 2048 levels, of a 12-bit capture is in the brightest
stop, 1024 in the next stop and so on until the lowest stop, which hsa 64
levels...

SO, based on this, the argument is to be as bright as possible without blowing
out the highlights. And that being 1 stop underexposed would cause a loss of
50% of the possible levels in the image...

My question is which is BETTER - to underexpose an image at a given ISO or to
increase the ISO and expose the image more brightly? For example - if I'm at
ISO 100 and my histogram is pushed toward the dark end - NOT clipped, just
"dark" am I better to increase the ISO to say 400 so my histogram is pushed
toward the bright end - AGAIN NOT CLIPPED...

Now to make things more interesting - in the RAW converter, I'd MOST LIKELY
increase the exposure of the "darker" image by 1 stop, and MOST LIKELY reduce
the exposure of the "brighter" image by 1 stop... So, in the end, IN THEORY,
both images would be identical once they got to Photoshop after conversion...

From what I understand, increasing the exposure on the "darker" image causes
more noise in the shadows... And the higher ISO has more noise everywhere...
But, is there a difference in the noise? Is one more obvious than the other?
Is one luminance noise versus color noise?

How about an even more extreme example - am I better leaving the ISO at 200 and
having the camera underexpose a shot by 1 stop (actual underexposure so I'm
perhaps clipping just a bit on the dark end) or bumping the ISO to 800, which I
KNOW causes visible noise and getting the exposure much brighter?

I"ll most likely run some tests just to see how things look in both cases, but
I'm curious what others have found...

dperez wrote:
>Of late I've seen a couple discussions of the importance of not underexposing
a
>digital image. Specifically this is because if you have 6 stops of dynamic
>range, half the data, or 2048 levels, of a 12-bit capture is in the brightest
>stop, 1024 in the next stop and so on until the lowest stop, which hsa 64
>levels...
>SO, based on this, the argument is to be as bright as possible without
>blowing
>out the highlights. And that being 1 stop underexposed would cause a loss of
>50% of the possible levels in the image...
>My question is which is BETTER - to underexpose an image at a given ISO or to
>increase the ISO and expose the image more brightly? For example - if I'm at
>ISO 100 and my histogram is pushed toward the dark end - NOT clipped, just
>"dark" am I better to increase the ISO to say 400 so my histogram is pushed
>toward the bright end - AGAIN NOT CLIPPED...
>Now to make things more interesting - in the RAW converter, I'd MOST LIKELY
>increase the exposure of the "darker" image by 1 stop, and MOST LIKELY reduce
>the exposure of the "brighter" image by 1 stop... So, in the end, IN THEORY,
>both images would be identical once they got to Photoshop after conversion...
>From what I understand, increasing the exposure on the "darker" image causes
>more noise in the shadows... And the higher ISO has more noise everywhere...
>But, is there a difference in the noise? Is one more obvious than the other?
>Is one luminance noise versus color noise?
>How about an even more extreme example - am I better leaving the ISO at 200
>and
>having the camera underexpose a shot by 1 stop (actual underexposure so I'm
>perhaps clipping just a bit on the dark end) or bumping the ISO to 800, which
>I
>KNOW causes visible noise and getting the exposure much brighter?
>I"ll most likely run some tests just to see how things look in both cases,
>but
>I'm curious what others have found...

Results are highly variable. You'll get better opinions if you identify what
camera you use. Different cameras have different abilities to pull image detail
out of shadows.

---Bob Gross---

dperez@juno_nospam.com wrote:
> Of late I've seen a couple discussions of the importance of not underexposing a
> digital image. Specifically this is because if you have 6 stops of dynamic
> range, half the data, or 2048 levels, of a 12-bit capture is in the brightest
> stop, 1024 in the next stop and so on until the lowest stop, which hsa 64
> levels...
>
> SO, based on this, the argument is to be as bright as possible without blowing
> out the highlights. And that being 1 stop underexposed would cause a loss of
> 50% of the possible levels in the image...
>
> My question is which is BETTER - to underexpose an image at a given ISO or to
> increase the ISO and expose the image more brightly? For example - if I'm at
> ISO 100 and my histogram is pushed toward the dark end - NOT clipped, just
> "dark" am I better to increase the ISO to say 400 so my histogram is pushed
> toward the bright end - AGAIN NOT CLIPPED...
>
> Now to make things more interesting - in the RAW converter, I'd MOST LIKELY
> increase the exposure of the "darker" image by 1 stop, and MOST LIKELY reduce
> the exposure of the "brighter" image by 1 stop... So, in the end, IN THEORY,
> both images would be identical once they got to Photoshop after conversion...
>
> From what I understand, increasing the exposure on the "darker" image causes
> more noise in the shadows... And the higher ISO has more noise everywhere...
> But, is there a difference in the noise? Is one more obvious than the other?
> Is one luminance noise versus color noise?
>
> How about an even more extreme example - am I better leaving the ISO at 200 and
> having the camera underexpose a shot by 1 stop (actual underexposure so I'm
> perhaps clipping just a bit on the dark end) or bumping the ISO to 800, which I
> KNOW causes visible noise and getting the exposure much brighter?
>
> I"ll most likely run some tests just to see how things look in both cases, but
> I'm curious what others have found...

If i can afford it I always leave mine at ISO 100 to get the lowest
possible noise. If you can keep it at ISO 100 and just change the
exposure to expose correctly do it. If not you should increase ISO
(after all thats what it is there for).
Even though your picture gets noisy you will still get the full dynamic
range of the sensor.
The noise that you are seing on a high iso setting is nothing more than
low levels amplified stronger. Since the noise always stays constant,
but the levels of your ccd output are lower you need more amplification
and that brings out more noise.
What you are proposing only shifts the problem to the computer for
amplification, but you do not eliminate the noise. So again bu
increasing exposure on the raw file you will essentialy end up with the
same result, but you will also have to live with your reduced dynamic
range (vs. the shot with higher iso)

Hope that was somewhat clear.

Andre

--
----------------------------------
http://www.aguntherphotography.com

dperez@juno_nospam.com wrote:
> Of late I've seen a couple discussions of the importance of not underexposing a
> digital image. Specifically this is because if you have 6 stops of dynamic
> range, half the data, or 2048 levels, of a 12-bit capture is in the brightest
> stop, 1024 in the next stop and so on until the lowest stop, which hsa 64
> levels...

> SO, based on this, the argument is to be as bright as possible
> without blowing out the highlights. And that being 1 stop
> underexposed would cause a loss of 50% of the possible levels in the
> image...

> My question is which is BETTER - to underexpose an image at a given
> ISO or to increase the ISO and expose the image more brightly?

It depends on the camera, but my answer is for high-end DSLRs.
Usually, such cameras have a "normal" range that goes from 100ish to
800ish ISO, and a "boost" range. Within the "normal" range, you
should set the ISO to what you need rather than deliberately
underexpose. Within the "boost" range I think it's just scaling up
the recorded image data, so it doesn't help much.

> For example - if I'm at ISO 100 and my histogram is pushed toward
> the dark end - NOT clipped, just "dark" am I better to increase the
> ISO to say 400 so my histogram is pushed toward the bright end -
> AGAIN NOT CLIPPED...

> Now to make things more interesting - in the RAW converter, I'd MOST
> LIKELY increase the exposure of the "darker" image by 1 stop, and
> MOST LIKELY reduce the exposure of the "brighter" image by 1
> stop... So, in the end, IN THEORY, both images would be identical
> once they got to Photoshop after conversion...

No, because the ISO setting in a DSLR can affect the gain before the
A-D convereter.

> How about an even more extreme example - am I better leaving the ISO
> at 200 and having the camera underexpose a shot by 1 stop (actual
> underexposure so I'm perhaps clipping just a bit on the dark end) or
> bumping the ISO to 800, which I KNOW causes visible noise and
> getting the exposure much brighter?

I think you'll get better results by setting the ISO appropriately.

> I"ll most likely run some tests just to see how things look in both
> cases, but I'm curious what others have found...

Interesting. Let us know.

Andrew.

In message <xolhd.3207$> ,
andre <> wrote:

>What you are proposing only shifts the problem to the computer for
>amplification, but you do not eliminate the noise. So again bu
>increasing exposure on the raw file you will essentialy end up with the
>same result, but you will also have to live with your reduced dynamic
>range (vs. the shot with higher iso)

Yes, but if you try to bring up the shadows in software, they get ugly
within 2 or 3 stops, and it's not just because of the noise. Pushing
ISO 100 3 stops to ISO 800 on my 10D is *much* uglier than using ISO 800
from the start. The reason is that the noise and shadow detail are
involved in intermodulation distortion from the quantization. At the
lower ISO range, I think very little image noise is readout/amplifier
noise, and going to ISO 200 or 400 (on a DSLR) to get the shutter speed
you need, and DOF you need, and saturate the RAW dynamic range at the
same time.
--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

wrote:
> The reason is that the noise and shadow detail are
> involved in intermodulation distortion from the quantization.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
What the heck does this mean?

Roger

dperez@juno_nospam.com wrote:

> Of late I've seen a couple discussions of the importance of not underexposing a
> digital image. Specifically this is because if you have 6 stops of dynamic
> range, half the data, or 2048 levels, of a 12-bit capture is in the brightest
> stop, 1024 in the next stop and so on until the lowest stop, which hsa 64
> levels...
>
> SO, based on this, the argument is to be as bright as possible without blowing
> out the highlights. And that being 1 stop underexposed would cause a loss of
> 50% of the possible levels in the image...
>
> My question is which is BETTER - to underexpose an image at a given ISO or to
> increase the ISO and expose the image more brightly? For example - if I'm at
> ISO 100 and my histogram is pushed toward the dark end - NOT clipped, just
> "dark" am I better to increase the ISO to say 400 so my histogram is pushed
> toward the bright end - AGAIN NOT CLIPPED...
>
> Now to make things more interesting - in the RAW converter, I'd MOST LIKELY
> increase the exposure of the "darker" image by 1 stop, and MOST LIKELY reduce
> the exposure of the "brighter" image by 1 stop... So, in the end, IN THEORY,
> both images would be identical once they got to Photoshop after conversion...
>
> From what I understand, increasing the exposure on the "darker" image causes
> more noise in the shadows... And the higher ISO has more noise everywhere...
> But, is there a difference in the noise? Is one more obvious than the other?
> Is one luminance noise versus color noise?
>
> How about an even more extreme example - am I better leaving the ISO at 200 and
> having the camera underexpose a shot by 1 stop (actual underexposure so I'm
> perhaps clipping just a bit on the dark end) or bumping the ISO to 800, which I
> KNOW causes visible noise and getting the exposure much brighter?
>
> I"ll most likely run some tests just to see how things look in both cases, but
> I'm curious what others have found...

A lot depends on your camera.
Here are two pages that might help give some relevant information:
http://clarkvision.com/imagedetail/d...ignal.to.noise

http://clarkvision.com/imagedetail/dynamicrange

I rarely change my metering pattern, but I do adjust the over/under
exposure based on the subject. I'll often set the iso low and meter
at -1 or even -2 stops when there are white subjects on dark backgrounds.
For example, see my bird gallery:

http://clarkvision.com/galleries/gallery.bird

and check out the images with white birds. The technical info is there
so you can see the results. Unless you have a low noise DSLR, you'll probably
not be able to do this, instead, you will need to get the highest
signal you can or the image signal to noise will be too low.
It is always best to maximize signal without blowing the highlights,
regardless of camera.

Roger

On Mon, 01 Nov 2004 19:05:08 -0700
In message <>
"Roger N. Clark (change username to rnclark)" <>
wrote:

> wrote:
> > The reason is that the noise and shadow detail are
> > involved in intermodulation distortion from the quantization.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> What the heck does this mean?

This means that the noise and coarse detail gets mixed together when
the numbers are added, multiplied, and divided. (He is mixing music
synthesis and radio terms to explain image manipultion...probably
because there is no clear way to explain it in technical terms.)

If the same photo had been shot at ISO 800 then more detail would have
visible despite the noise. Kinda like the difference between punching
up an old 78 record vs listening to a 11kHz CD.

Jeff

Confused wrote:

> On Mon, 01 Nov 2004 19:05:08 -0700
> In message <>
> "Roger N. Clark (change username to rnclark)" <>
> wrote:
>
>
>> wrote:
>>
>>>The reason is that the noise and shadow detail are
>>>involved in intermodulation distortion from the quantization.
>>
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>What the heck does this mean?
>
>
> This means that the noise and coarse detail gets mixed together when
> the numbers are added, multiplied, and divided. (He is mixing music
> synthesis and radio terms to explain image manipultion...probably
> because there is no clear way to explain it in technical terms.)
>
> If the same photo had been shot at ISO 800 then more detail would have
> visible despite the noise. Kinda like the difference between punching
> up an old 78 record vs listening to a 11kHz CD.
>
> Jeff

from: http://www.its.bldrdoc.gov/fs-1037/dir-019/_2813.htm

intermodulation distortion: Nonlinear distortion characterized
by the appearance, in the output of a device, of frequencies that
are linear combinations of the fundamental frequencies and all
harmonics present in the input signals. (18 Note: Harmonic
components themselves are not usually considered to characterize
intermodulation distortion. When the harmonics are included as
part of the distortion, a statement to that effect should be made.

This HTML version of FS-1037C was last generated on Fri Aug 23 00:22:38 MDT 1996

"intermodulation distortion from the quantization" is jargon that
seems meaningless. There is no intermodulation in this case. There
are no linear combination of frequencies. There are no harmonic
components. It is simple quantization due to the use of integer
numbers. The general digital photography term is "posterization."
If more bits were used in processing, like scaled 16-bit values
(scaled up from 12-bit camera numbers) posterization can be reduced
usually to the point of being negligible. This is discussed on
my dynamic range page:

http://clarkvision.com/imagedetail/dynamicrange

Roger

On Mon, 01 Nov 2004 20:52:26 -0700
In message <>
Roger N. Clark wrote:

> Confused wrote:
>
> > On Mon, 01 Nov 2004 19:05:08 -0700
> > In message <>
> > "Roger N. Clark (change username to rnclark)" <>
> >
> >> wrote:
> >>
> >>>The reason is that the noise and shadow detail are
> >>>involved in intermodulation distortion from the quantization.
> >>
> >> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Roger trolled:

> >>What the heck does this mean?

I took the bait. Ironically, I hit the nail exactly on the head,
only based on my background with additive and FM digital
synthesis in the 70's, a SW radio monitoring hobby, a career
in software engineering, and my take on trying to describe something
that is very confusing.

But I try and see the simple answers. In this case, there is both an
objective and subjective similarity in the development of both audio
and visual reproduction in a quickly changing digital environment.

> > This means that the noise and coarse detail gets mixed together when
> > the numbers are added, multiplied, and divided. (He is mixing music
> > synthesis and radio terms to explain image manipultion...probably
> > because there is no clear way to explain it in technical terms.)

See (1)

> > If the same photo had been shot at ISO 800 then more detail would have
> > visible despite the noise. Kinda like the difference between punching
> > up an old 78 record vs listening to a 11kHz CD.

See (2)

> > Jeff

(1.a)

Thanks for making my point. However, I wouldn't consider a government
agency involved in radio and audio transmission an authority on image
processing as related to state of the art photographic manipulation.
The technical details are tightly held by a small number of very
competitive companies, and any related up to date information held by
a government would be classified. We the people are left to our best
interpretation of limited and filtered information.

http://www.its.bldrdoc.gov/

"ITS is the research and engineering branch of the
National Telecommunications and Information Administration (NTIA,
a part of the U.S. Department of Commerce (DOC)."

But then, the DOC/NTIA/ITS is involved with audio and radio
engineering, so, again, thanks for validating my comment!


> from: http://www.its.bldrdoc.gov/fs-1037/dir-019/_2813.htm
>
> intermodulation distortion: Nonlinear distortion characterized
> by the appearance, in the output of a device, of frequencies that
> are linear combinations of the fundamental frequencies and all
> harmonics present in the input signals. (18 Note: Harmonic
> components themselves are not usually considered to characterize
> intermodulation distortion. When the harmonics are included as
> part of the distortion, a statement to that effect should be made.
>
> This HTML version of FS-1037C was last generated on Fri Aug 23 00:22:38 MDT 1996
>
> "intermodulation distortion from the quantization" is jargon that
> seems meaningless. There is no intermodulation in this case. There
> are no linear combination of frequencies.

(1.b)

Linearity is not a requirement of "intermodulation"... an
understandable mistake in a small focused sub-agency of a government
commerce department. If one were to dig deep enough my statement
could be made to look false. However, frequency modulation, and
intermodulation, is not dependent on any concept of linearity. The
intermodulation of frequencies in music is usually non-linear, and
when computers enter the picture quantization is a constant problem
when realizing sound, just as it is in realizing light reflection.
The sheer number of variables eliminates linearity of distortion
(modulation, intermodulation, anyothertypeofmodulation of the data at
hand).

> There are no harmonic
> components. It is simple quantization due to the use of integer
> numbers. The general digital photography term is "posterization."
> If more bits were used in processing, like scaled 16-bit values
> (scaled up from 12-bit camera numbers) posterization can be reduced
> usually to the point of being negligible. This is discussed on
> my dynamic range page:
>
> http://clarkvision.com/imagedetail/dynamicrange

(2)

Great examples of ancient 78's and undersampled audio vs photographic
imaging. Neither sounds good, neither looks good; all garbage. The
graphs are kinda spiffy... what do they mean? It looks like they are
saying, "We have to wait for technology to advance another year or
three before we will be happy with the dynamic range of digital
scanners and cameras."

Jeff