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Under-exposure Versus Higher ISO?

 
 
Mardon
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      05-05-2006
Suppose that I'm using a Canon 20D and shooting in RAW, with the
slowest shutter speed and largest aperture that I can use for a given
situation. I'm at ISO800 ant the photo is still being under-exposed.
How bad must the under-exposure become before it's better to up the
ISO to 1600? Same question if I'm already at 1600. How bad must the
under-exposure be before I'm better off to up the ISO to 3200?

I know that the answer to this question can be complicated by
suggesting flash and other such things to increase the light but
that's not my point. I'm hoping to get a simple response about the
final picture quality that can be achieved when evaluating the trade-
off between under-exposing versus using a higher ISO while shooting
RAW.
 
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Ed Ruf
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      05-05-2006
On Fri, 05 May 2006 14:54:35 GMT, in rec.photo.digital Mardon
<(E-Mail Removed)> wrote:

>Suppose that I'm using a Canon 20D and shooting in RAW, with the
>slowest shutter speed and largest aperture that I can use for a given
>situation. I'm at ISO800 ant the photo is still being under-exposed.
>How bad must the under-exposure become before it's better to up the
>ISO to 1600? Same question if I'm already at 1600. How bad must the
>under-exposure be before I'm better off to up the ISO to 3200?


I believe it's always better to properly expose with higher iso to
increase the signal to noise ratio. I think it is then easier to
filter the noise. One thing to consider is are the extreme higher isos
real. Or are they just iso1600 pushed with analog amplification? That
is the case with my D200 above 1600. I recently used 3200 to get some
shots of some herons doing a mating dance when I had no other
alternative. I didn't have time to try 1600. This was shot across a
creek at twilight with a 200mm lens + 2x TC, so I needed everything I
could to get shutter speeds up. I haven't made a 1:1 comparison of
1600 lighted later vs 3200 yet, so I haven't got any hard evidence of
which is better.
__________________________________________________ ______
Ed Ruf Lifetime AMA# 344007 ((E-Mail Removed))
http://EdwardGRuf.com
 
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All Things Mopar
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      05-05-2006
Today Mardon commented courteously on the subject at hand

> Suppose that I'm using a Canon 20D and shooting in RAW,
> with the slowest shutter speed and largest aperture that I
> can use for a given situation. I'm at ISO800 ant the photo
> is still being under-exposed. How bad must the
> under-exposure become before it's better to up the ISO to
> 1600? Same question if I'm already at 1600. How bad must
> the under-exposure be before I'm better off to up the ISO
> to 3200?


I can't speak to the 20D but in the case of my Rebel XT, I
would stay at ISO 800 because the Rebel is quite noisy at that
"speed" and is useless to me at 1600.

So, without your saying what you percieve your 20D noise to be
at 400, 800, 1600, and 3200, isn't this a theoretical question
that you could answer by taking some controlled test shots
varying not only the ISO but appropriate other photometric
settings?

> I know that the answer to this question can be complicated
> by suggesting flash and other such things to increase the
> light but that's not my point. I'm hoping to get a simple
> response about the final picture quality that can be
> achieved when evaluating the trade- off between
> under-exposing versus using a higher ISO while shooting
> RAW.


Yes, flash would not only complicated the issue, it would
invalidate it unless you were intending to expose the main
subject with the flash and slave(s) if any, and using the high
ISO to bring up the background exposure.

But, in the final analysis, it is yours, and only your opinion
that matters, given the type of subject(s) you shoot, what
dynamic range and DOF you wish to achieve, and your tolerance
for noise.

--
ATM, aka Jerry

"Go ahead, make my day", Dirty Harry in the movie "Sudden
Impact"
 
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nikojorj_jaimepaslapub@yahoo.Fr
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      05-05-2006

Ed Ruf wrote:
> One thing to consider is are the extreme higher isos
> real. Or are they just iso1600 pushed with analog amplification?


They're not really real : they're digital ;o).

A sensor has only one 'real' sensibility (generally the lower one), the
others are made by a signal amplification - that, more or less,
amplifies also noise.

The only differences i can see are :
- some S/N optimization algorithm may also be applied after
amplification, but would it be better in-camera than what you can do in
post-processing with those neat ninjas of denoiser softs?
- if the analog/digital conversion is made after amplification (likely,
isn't it?), the higher-ISO signal contains more signal levels than the
underexposed one... but I'd think that in the high ISO (and 16-bit)
case, both are limited by noise?

So, my personal conclusion would be 'six of one and half a dozen of the
other' between underexposing and cranking ISO setting (while RAW of
course)...
Any different input would be of course gladly welcome!

 
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Mardon
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      05-05-2006
All Things Mopar <(E-Mail Removed)> wrote:

> I can't speak to the 20D but in the case of my Rebel XT, I
> would stay at ISO 800 because the Rebel is quite noisy at that
> "speed" and is useless to me at 1600.


The 20D is quite usable at ISO1600 but some noise is certainly
visible. ISO3200 works on the 20D but it is too noisy for most
purposes.

> So, without your saying what you percieve your 20D noise to be
> at 400, 800, 1600, and 3200, isn't this a theoretical question
> that you could answer by taking some controlled test shots
> varying not only the ISO but appropriate other photometric
> settings?


I was hoping that someone had already performed the sort of
'experiment' that you describe and that they would post their results
in response to my question. Performing these theoretical tests that
require controlling several variable combinations isn't really 'my
thing'. My impression based on my everyday experience is that if a
RAW image is underexposed by more than a full f stop, then it's
probably better to go to the higher ISO. If the underexposure is
less than a full stop, I think it's better to shoot at the lower ISO
and use Noise Ninja or some other noise filter to reduce the noise.
(BTW, I find Noise Ninja better than the noise reduction filter built
into PS CS2.) I'm not sure of this conclusion , however, and wanted
to find out what others think.
 
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Scott W
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      05-05-2006
http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> Ed Ruf wrote:
> > One thing to consider is are the extreme higher isos
> > real. Or are they just iso1600 pushed with analog amplification?

>
> They're not really real : they're digital ;o).
>
> A sensor has only one 'real' sensibility (generally the lower one), the
> others are made by a signal amplification - that, more or less,
> amplifies also noise.
>
> The only differences i can see are :
> - some S/N optimization algorithm may also be applied after
> amplification, but would it be better in-camera than what you can do in
> post-processing with those neat ninjas of denoiser softs?
> - if the analog/digital conversion is made after amplification (likely,
> isn't it?), the higher-ISO signal contains more signal levels than the
> underexposed one... but I'd think that in the high ISO (and 16-bit)
> case, both are limited by noise?
>
> So, my personal conclusion would be 'six of one and half a dozen of the
> other' between underexposing and cranking ISO setting (while RAW of
> course)...


A quick test with a Rebel XT shows a lot less noise in the ISO 400 shot
then the 100, when both are taken at the same aperture and shutter
speeds.

So at least between iso 100 and 400 it is much better to use iso 400
then simply underexpose

Scott

 
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ASAAR
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      05-05-2006
On Fri, 05 May 2006 11:03:25 -0400, Ed Ruf wrote:

>> Suppose that I'm using a Canon 20D and shooting in RAW, with the
>> slowest shutter speed and largest aperture that I can use for a given
>> situation. I'm at ISO800 ant the photo is still being under-exposed.
>> How bad must the under-exposure become before it's better to up the
>> ISO to 1600? Same question if I'm already at 1600. How bad must the
>> under-exposure be before I'm better off to up the ISO to 3200?

>
> I believe it's always better to properly expose with higher iso to
> increase the signal to noise ratio. I think it is then easier to
> filter the noise. One thing to consider is are the extreme higher isos
> real. Or are they just iso1600 pushed with analog amplification?


Why would that be? I'd think that you'd have more to work with if
the lowest ISO was used. Consider a shot taken at ISO 100 that
requires for proper exposure a shutter speed of 1/60 sec at f/4. If
the same shot is taken with the camera set to ISO 400, the camera
might use 1/250 sec at f/4 or 1/60 sec at f/8, but the final image
will appear to be the same, still underexposed, but with the
addition of a bit of added noise. But the first shot captured more
light. Four times as many photons, so wouldn't you be able to
manipulate the RAW file with software to bring out more of the
shadow detail, which isn't buried in as much noise? Continuing
along this line, there could be a problem with highlights in the
same shot being blown, as more individual sensor cells might have
filled to their capacity. But this would be more of a problem if
the original problem was overexposure, not underexposure.

Note: I'm basing this on the OP's saying that he's using the slowest
shutter speed and largest aperture "for a given situation", and that
means that the limit is his, not the camera's. For instance, the
camera might be capable of using f/1.4, but he isn't willing to
sacrifice DOF by opening beyond f/4, or the camera can use a shutter
speed of 1/'15 sec, but that wouldn't be fast enough to eliminate
subject movement. It could be a different matter if the limitation
was the camera's. For example, if he's using shutter priority mode
because he requires a shutter speed of no slower than 1/60 sec for a
particular shot, but at that speed a proper exposure would require
an f/2 lens but the maximum available aperture is only f/4,
underexposure would be the result. But in this case, a higher ISO
would allow for a proper exposure.

Your example of the twilight heron shots seems to be an example of
the latter case, where you required a fast shutter speed and the
camera's lens probably didn't have a wide enough aperture for the
available light, so the higher ISO was appropriate. So while the
shutter speed was determined by you (not limited by the camera),
with the aperture, you ran into a physical limit of the lens.

Whether I'm right or wrong about this, I hope I've explained it
clearly enough to be followed, and whether I'm right also depends on
my understanding of whether the OP's exposure limits are created by
his requirements, or if they are limited by the camera's hardware.

 
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Mardon
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      05-05-2006
ASAAR <(E-Mail Removed)> wrote:

> Whether I'm right or wrong about this, I hope I've explained
> it
> clearly enough to be followed, and whether I'm right also
> depends on my understanding of whether the OP's exposure limits
> are created by his requirements, or if they are limited by the
> camera's hardware.


I posted the question in the context of doing macro photos of live
'bugs' that are 1 mm to 3 mm long. The lens is an f/3.5 180mmL and
the camera a 20D. I have to keep the shutter speed up because the
bugs are alive and moving. I can't open the apature too much because
there is almost no depth of field anyway with these extreme close-
ups. The lens can stop down to f/45 but I prefer to shoot the
'bugs' in natural light and f/45 at 1/250 sec just doesn't give
enough light even at ISO1600. I generally wind up at about ISO1600
f/22 at 1/250 or 1/320. I can use ISO800 but it underexposes. That
is the basis of my question.
 
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All Things Mopar
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      05-05-2006
Today Mardon attempted to dazzle everyone with this profound
linguistic utterance

> The 20D is quite usable at ISO1600 but some noise is
> certainly visible. ISO3200 works on the 20D but it is too
> noisy for most purposes.


How would you judge 20D noise vs. Rebel XT? I am beginning to
think I shoulda popped the bucks for the 20D and just suffered
with the larger size and weight. My available light tests show
that ISO 400 is the highest I like, I can deal with 800 with
lots of work, but 1600 is useless (to me).

>> So, without your saying what you percieve your 20D noise
>> to be at 400, 800, 1600, and 3200, isn't this a
>> theoretical question that you could answer by taking some
>> controlled test shots varying not only the ISO but
>> appropriate other photometric settings?

>
> I was hoping that someone had already performed the sort of
> 'experiment' that you describe and that they would post
> their results in response to my question. Performing these
> theoretical tests that require controlling several variable
> combinations isn't really 'my thing'. My impression based
> on my everyday experience is that if a RAW image is
> underexposed by more than a full f stop, then it's probably
> better to go to the higher ISO. If the underexposure is
> less than a full stop, I think it's better to shoot at the
> lower ISO and use Noise Ninja or some other noise filter to
> reduce the noise. (BTW, I find Noise Ninja better than the
> noise reduction filter built into PS CS2.) I'm not sure of
> this conclusion , however, and wanted to find out what
> others think.
>

I understand what a "controlled test" is and what it involves.
I have used "design of experiments" techniques to do digital
camera testing, principally when evaluating a Nikon 8400 last
year, which I took back, and my current Rebel XT which I kept.

One f/stop is only half as bright, which is easily corrected.
Yes, it will produce noise, but not nearly as much as a 2,3,5
stop under, as often happens.

The opinions/experience of others can be valuable, but hardly
definitive. e.g., whether this is "your thing" or not, how
would you know what my results are or anyone elses without
knowing what "controlled conditions" any of us used? Besides
the variables you already discussed, and my clarifying
comments and questions, personal preference is the real
deciding factor, but subject type and the full gamut of
lighting is also of paramount importance.

Then, there's the general issue of hand-held vs. tripod or
monopod ...

--
ATM, aka Jerry

"I came, I saw, I conquered" - Alexander The Great
 
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All Things Mopar
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      05-05-2006
Today Mardon attempted to dazzle everyone with this profound
linguistic utterance

> I posted the question in the context of doing macro photos
> of live 'bugs' that are 1 mm to 3 mm long. The lens is an
> f/3.5 180mmL and the camera a 20D. I have to keep the
> shutter speed up because the bugs are alive and moving. I
> can't open the apature too much because there is almost no
> depth of field anyway with these extreme close- ups. The
> lens can stop down to f/45 but I prefer to shoot the 'bugs'
> in natural light and f/45 at 1/250 sec just doesn't give
> enough light even at ISO1600. I generally wind up at about
> ISO1600 f/22 at 1/250 or 1/320. I can use ISO800 but it
> underexposes. That is the basis of my question.


I must've missed this part. There is a world of difference
between macro anything and general photography, or specialized
subjects such as sports, architectural, portaits, fine detail
such as cars or objects d'art, etc. using the same camera.

--
ATM, aka Jerry

"I came, I saw, I conquered" - Alexander The Great
 
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