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