Re: D700 sample photos up to 25,600 ISO

"Wilba" <usenet@xxxxxxxxxxxxxxxxxxx> wrote:
Floyd L. Davidson wrote:

With multiple exposures it is possible to get (using 8 exposures)
either the effect of 8 times the ISO or the noise equivalent of a
significantly lower ISO. By setting the multiple exposures to be
averaged for the output, the noise is greatly reduced while the
brightness stays the same, and hence an ISO 25,600 image
with noise closer to what would have been produced at ISO
3200 is produced.

My camera doesn't do multiple exposures, so I've been playing around with
this idea in Photoshop (CS3). I adjusted the exposure of a raw file
to -1, -2, -3, and -4 steps, and used duplicate layers in each case with the
blending mode set to Screen ("The effect is similar to projecting multiple
photographic slides on top of each other.")

Unfortunately that doesn't quite provide the same thing
as multiple exposures. You can do it in software
though, but you need to actually take multiple images
rather than use just the same RAW data.

Using just one exposure means that noise will be in the
same locations. Some types of manipulation might change
the amount of noise slightly, but not nearly to the same
effect that random distribution between different
exposures.

The whole trick to averaging out noise is that the noise
will never be exactly the same for two otherwise
identical exposures. The desired signal will be very
close to identical though. For the desired signal the
average will be almost exactly the same as any one of
the single images. But the noise, because it is
randomly spread around, will average out to be less at
any given location than the maximum noise that was in
any one of those images. That means that the number of
pixels that have noise won't change, but the average
maximum value of noise will be less.

As expected, the -1 exposure needs two layers to look about right (similar
to the unadjusted exposure), but -2 needs 3 layers, not 4. -3 needs 5
layers, not 8, and -4 needs 9.

The relationship between the underexposure and number and layers doesn't
make sense to me. I guess it's about the blending mode and how PS does it.
Thoughts?

I'm not sure what that might be. First, I don't use
PhotoShop at all, so I'm not even vaguely familiar with
how it adds layers. (I assume it is like GIMP and can
add two layers in many many ways, each of which can be
infinitely adjusted...) Second, it takes more than just
what would be obvious to know what it is doing. You
could experiment a bit though...

Take a greyscale, put it into two layers, and add it the
same way you are adding those images. Then see what the
resulting levels are in each band (averaged over some
large enough area to get a valid answer). For example,
if every band with a value of 128 or higher becomes 255,
and a band with the value of 64 becomes 128... then you
know it really is simple addition. If it is something
else, that will be obvious... more or less. For example
it might be that 64 + 64 turns out to be not 2 * 64, but
rather the square root of 2 times 64, 1.414 * 64.

Also, the "-1 exposure" might not actually be the same
as 1 fstop low. And if the image values are gamma
adjusted, I personally have no desire to sit down and do
the calculation! :-)

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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