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"Exposure" vs "Digitization



 
 
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  #41  
Old August 10th 05, 11:58 PM
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In message ,
"Bart van der Wolf" wrote:

wrote in message
.. .
SNIP
The rule of "using the lowest ISO" for maximum capture quality
only works when DOF and shutter speed are not issues (static
scene with tripod and MLU).


True, but for me that's automatically solved when I "expose to the
right".


Yes, but do you expose to the right *and* raise the ISO to get the
motion/DOF parameters you need? I think only a minority of people truly
understand that going to a higher ISO is not only sometimes necessary,
but it is also *NOT* necessarily a quality compromise; not unless you
clip the data or lower the absolute sensor exposure witha faster shutter
speed and/or a smaller aperture.

Let's say that you're shooting a black subject against a middle-grey
background with a 100mm lens on a full-frame DSLR, at a distance, and
you want the DOF that is had at f8. The camera's metering tells you
that at ISO 100 and 1/100 and f/8, you are "under-exposed" (or, in my
suggested terms, "under-digitized") by one stop. Common wisdom would
dictate to most people that they need to move to ISO 200. The fact is,
you could move to ISO 800, maybe even 1600, with cleaner, more detailed
results, with the same aperture and shutter speed (for 1600 you might
need to decrease the absolute exposure just a tad to avoid blowing out
the green channel).

Anyone who thinks that what I just wrote is outrageous is clearly
operating in an inefficent exposure/digitization paradigm.

I like to look at things with efficient models, and nothing is clearer
than looking at ISO choice in terms of getting the most output range for
a given absolute sensor exposure.

It isn't ISO per se that causes noise; it is the S/N ratio in the
absolute analog sensor exposure that determines the starting noise, and
ISO has absolutely no effect on that exposure except in how it affects
the camera's metering. It has an effect on how that exposure is
digitized, and at this stage, the higher the ISO is, the less noise
there is in the image, because there is less quantization. Of course,
this assumes analog gains proportional to the ISO numbers.

If the shutterspeed and aperture are cast in concrete (which
they often aren't), then all that's left is to correct with the ISO
setting (not to influence the exposure meter, I'd probably use Manual
in such a situation) in order to change the amplification on the
analog signal before ADC.


Too bad this couldn't be done in finer increments than one stop, on most
cameras. You have to vary f-stop and shutter speed by 1/2 or 1/3 stop
to get the in-between levels of digitization.

However, if capturing the full scene Dynamic Range is important, I'd
probably choose ISO 100 if I need to get the best sensor DR output,
and adjust the Depth-of-Field / camera shake / subject motion
trade-off.


I don't know what camera you have, but I really can't see much of a
difference in image quality between 100 and 200 with my 20D. It seems
to me that 200 has just enough noise to dither the lower bits into
softening the quantization that happens at ISO 100. So, you get
slightly more random noise with 200, and slightly more posterization
effect with 100, and they come up about equal. That one stop can often
go a long way towards a better shutter speed, or get you away from the
poor "wide-open" optics that many lenses have. IS0 400 is noticeably
poorer, and from there up the difference becomes more dramatic with each
doubling of the ISO setting. This, of course, is in reference to shadow
quality. High-key images do not vary as much at the various ISOs.
--


John P Sheehy

  #42  
Old August 11th 05, 12:15 AM
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In message ,
Alan Browne wrote:

Yes, I owuld guess that it is so. When below, eg, ISO 1600, the ADC is
stepped, and at 1600 and above the sensor readings are left shifted.
There may or may not be an overlapped zone where some gain is in the ADC
and some gain is in shifting left.

Then, there may be some filtering in the ADC as well as (of course)
making the RGB image from the Bayer image that will disgusise (filter,
badly or otherwise) the quantization effects.


The dynamics of the noise are greater than the dynamics of quantization
at the highest ISOs. The lower ISOs are the ones that show the most
quantization effect.
--


John P Sheehy

  #43  
Old August 11th 05, 12:23 AM
Jeremy Nixon
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wrote:

Let's say that you're shooting a black subject against a middle-grey
background with a 100mm lens on a full-frame DSLR, at a distance, and
you want the DOF that is had at f8. The camera's metering tells you
that at ISO 100 and 1/100 and f/8, you are "under-exposed" (or, in my
suggested terms, "under-digitized") by one stop. Common wisdom would
dictate to most people that they need to move to ISO 200. The fact is,
you could move to ISO 800, maybe even 1600, with cleaner, more detailed
results, with the same aperture and shutter speed (for 1600 you might
need to decrease the absolute exposure just a tad to avoid blowing out
the green channel).

Anyone who thinks that what I just wrote is outrageous is clearly
operating in an inefficent exposure/digitization paradigm.


It's not outrageous at all -- it just bears no resemblance to any way
I've ever approached photography. I guess it makes sense in the above
situation if you actually believed the camera's light meter, which
was wildly incorrect; if the better exposure was at ISO 800, then the
meter was two full stops off in its judgement, because you weren't
underexposing by one stop at ISO 100, you were underexposing by three
stops. So you're still comparing a proper exposure at ISO 800 to a
two-stops-under exposure at ISO 200, and the moral of the story is
still that it's better to boost ISO than to underexpose.

Too bad this couldn't be done in finer increments than one stop, on most
cameras. You have to vary f-stop and shutter speed by 1/2 or 1/3 stop
to get the in-between levels of digitization.


Do the Canons really make you use full-stop increments??

--
Jeremy |
  #44  
Old August 11th 05, 12:23 AM
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In message ,
Alan Browne wrote:

Quantization noise is further disguised in character, of course, by the
conversion from bayer to RGB.


Not if you look directly at the RAW data.

Demosaicing and white-balancing certainly hide a multitude of
quantization sins. If a camera had no CFA, and was greyscale, then an
8-bit Tiff with a 2.2-gamma-adjusted output from a greyscale RAW file
would have values of something like 0, 9, 16, 20, 22, etc, in the
deepest shadows, with nothing in-between. The WB and demosaicing create
all kinds of intermediate values that fool a histogram, but doesn't have
quite the shadows that it could, if the camera's sensor "sensed" in a
gamma-adjusted (and already white-balanced) manner.
--


John P Sheehy

  #45  
Old August 11th 05, 12:47 AM
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In message ,
Jeremy Nixon wrote:

wrote:


Let's say that you're shooting a black subject against a middle-grey
background with a 100mm lens on a full-frame DSLR, at a distance, and
you want the DOF that is had at f8. The camera's metering tells you
that at ISO 100 and 1/100 and f/8, you are "under-exposed" (or, in my
suggested terms, "under-digitized") by one stop. Common wisdom would
dictate to most people that they need to move to ISO 200. The fact is,
you could move to ISO 800, maybe even 1600, with cleaner, more detailed
results, with the same aperture and shutter speed (for 1600 you might
need to decrease the absolute exposure just a tad to avoid blowing out
the green channel).


Anyone who thinks that what I just wrote is outrageous is clearly
operating in an inefficent exposure/digitization paradigm.


It's not outrageous at all -- it just bears no resemblance to any way
I've ever approached photography. I guess it makes sense in the above
situation if you actually believed the camera's light meter, which
was wildly incorrect; if the better exposure was at ISO 800, then the
meter was two full stops off in its judgement, because you weren't
underexposing by one stop at ISO 100, you were underexposing by three
stops. So you're still comparing a proper exposure at ISO 800 to a
two-stops-under exposure at ISO 200, and the moral of the story is
still that it's better to boost ISO than to underexpose.


You misunderstood; there is nothing wrong with the metering in this
scenario. The scene had the approximate reflectivity of a grey card.

Too bad this couldn't be done in finer increments than one stop, on most
cameras. You have to vary f-stop and shutter speed by 1/2 or 1/3 stop
to get the in-between levels of digitization.


Do the Canons really make you use full-stop increments??


For ISO? Yes, the ISOs are 100, 200, 400, 800, 1600, 3200 On the
non-1-series cameras.
--


John P Sheehy

  #46  
Old August 11th 05, 02:11 AM
Jeremy Nixon
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wrote:

You misunderstood; there is nothing wrong with the metering in this
scenario. The scene had the approximate reflectivity of a grey card.


No, there was; if the meter indicates one stop underexposure, but you can
increase three stops and end up with a proper exposure, then the meter was
actually off by two stops. You're comparing "trust the meter" at ISO 200
with "don't trust the meter" at ISO 800, and that's not a useful
comparison, because if you trusted the meter in the first situation
you'd trust it in the second and still underexpose by two stops, but if
you weren't going to trust the meter in the first place (and thus be
willing to go up two more stops from what it said at ISO 800) then you'd
have been willing to do so at ISO 200, too, if you could -- leaving the
choices being either underexpose at 200 or expose properly at 800.

Do the Canons really make you use full-stop increments??


For ISO? Yes, the ISOs are 100, 200, 400, 800, 1600, 3200 On the
non-1-series cameras.


Ah, so it's one of those "held back" features. Annoying how they use
(seemingly) simple features to convince you to spend more money.

--
Jeremy |
  #47  
Old August 11th 05, 09:50 AM
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In message ,
Jeremy Nixon wrote:

wrote:

You misunderstood; there is nothing wrong with the metering in this
scenario. The scene had the approximate reflectivity of a grey card.


No, there was; if the meter indicates one stop underexposure, but you can
increase three stops and end up with a proper exposure,


What is "proper exposure"?

then the meter was
actually off by two stops.


No, the meter is dumb, and it puts the mid-key scene at middle grey,
which is not *optimum*, unless your goal is to use a JPEG as-is, or
print directly from the camera. In terms of RAW capture, it is a
relatively poor digitization and/or exposure.

You're comparing "trust the meter" at ISO 200
with "don't trust the meter" at ISO 800, and that's not a useful
comparison, because if you trusted the meter in the first situation
you'd trust it in the second and still underexpose by two stops, but if
you weren't going to trust the meter in the first place (and thus be
willing to go up two more stops from what it said at ISO 800) then you'd
have been willing to do so at ISO 200, too, if you could -- leaving the
choices being either underexpose at 200 or expose properly at 800.


I clearly stated that we were a stop under because of *NEEDED* f-stop
and shutter speed! You're going to blur the image now?

Let me try another way. Let's pretend the camera is greyscale (no CFA),
to simplify matters. The camera meters a grey card (or the ~18% grey
scene) as if to wind up with an average RAW value of 400 (fairly
typical) out of about 4000 possible levels. At ISO 100, you fall a stop
short of that with your needed f-stop and shutter speed, so the RAW
output would be 200. Using the same aperture and shutter speed (as
needed), with ISO 800, the average now is a RAW value of 1600. If you
used ISO 1600, the average RAW value would be 3200, and you might have a
few brighter spots clipping past 4095.

--


John P Sheehy

  #48  
Old August 11th 05, 08:05 PM
Jeremy Nixon
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wrote:
Jeremy Nixon wrote:

No, there was; if the meter indicates one stop underexposure, but you can
increase three stops and end up with a proper exposure,


What is "proper exposure"?


The exposure you want is the proper exposure. In this case, +2 from the
meter reading.

then the meter was actually off by two stops.


No, the meter is dumb, and it puts the mid-key scene at middle grey,
which is not *optimum*, unless your goal is to use a JPEG as-is, or
print directly from the camera. In terms of RAW capture, it is a
relatively poor digitization and/or exposure.


Right. The meter is off by two stops from the exposure you want.

You're comparing "trust the meter" at ISO 200
with "don't trust the meter" at ISO 800, and that's not a useful
comparison, because if you trusted the meter in the first situation
you'd trust it in the second and still underexpose by two stops, but if
you weren't going to trust the meter in the first place (and thus be
willing to go up two more stops from what it said at ISO 800) then you'd
have been willing to do so at ISO 200, too, if you could -- leaving the
choices being either underexpose at 200 or expose properly at 800.


I clearly stated that we were a stop under because of *NEEDED* f-stop
and shutter speed! You're going to blur the image now?


No. Using the needed f-stop and shutter speed, at ISO 200 you are
underexposing, and at 800 you are not. You're suggesting that at
ISO 200 you are blindly trusting the meter, but once you go to 800
you suddenly aren't and are willing to use +2 EC from what it's
telling you to get a good exposure.

In real life, if someone is willing to use +2 EC to get a good exposure
at ISO 800, then they were willing to do so at ISO 200, and the only
reason they didn't is because the shutter speed would end up too low.
So, the conclusion is that there isn't enough light for ISO 200, and
the shot would be underexposed. Since it's better to boost the ISO
than to underexpose, you boost the ISO.

The properly exposed shot at ISO 800 will be of better technical quality
than the underexposed one at 200. However, the properly exposed shot
at 800 will have *more* noise than the properly exposed shot at ISO 200;
it's just that you couldn't get the shot at 200.

The usual thought process here, from the photographer's perspective,
would be: good exposure is at +2 EC. I can't get there with shutter
speed because it'll be too slow to hand-hold; I can't get there with
aperture because I need the depth of field; so I get there with ISO.

Let me try another way. Let's pretend the camera is greyscale (no CFA),
to simplify matters. The camera meters a grey card (or the ~18% grey
scene) as if to wind up with an average RAW value of 400 (fairly
typical) out of about 4000 possible levels. At ISO 100, you fall a stop
short of that with your needed f-stop and shutter speed, so the RAW
output would be 200. Using the same aperture and shutter speed (as
needed), with ISO 800, the average now is a RAW value of 1600. If you
used ISO 1600, the average RAW value would be 3200, and you might have a
few brighter spots clipping past 4095.


Right. At ISO 100, you're drastically underexposing. At 200, you're
still underexposing. At 800, you're not. At 1600, you're overexposing,
unless you intended for those bright spots to clip (spectral reflections,
point light sources, etc).

--
Jeremy |
 




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