> I have a fairly advanced P&S camera - Canon A630.
>
> I can persuade it to do RAW mode via CHDK
>
> http://scratchpad.wikia.com/wiki/CHDK
>
> I could also use CHDK to take shutter speed
> bracketed shots for HDR usage,
> although this only applies to
> static images, which restricts me somewhat.
>
> I note that it is common to use a grad ND filter
> to reduce burnout in the sky, on "typical"
> landscape shots.
>
> Could I use an ND grad filter, and raw mode,
> and post-apply a correction, so as to
> increase the dynamic range of the image?
>
> This trick would be a pseudo HDR,
> using different exposures (via the grad filter)
> in different parts of the image.
>
> I could then use "normal" HDR tone mapping
> to get my desired final image.
>
> Comments, or hints from people that have already
> done this would be most welcome.
>
Short answer: Yes.
There are many techniques you can use to extend the dynamic range of a
digital camera, and the simplest is probably the hangover from film
usage, the ND grad filter, which darkens the upper part of the frame,
which is usually the bright sky in landscape shots. You can get
different types of ND grads, with different levels of attenuation (Cokin
do ND2, ND4, and ND8), and have "hard" or "soft" transitions from the
clear to dark sections - hard is probably best for compact cameras.
There are some ND grads in circular screw in mounts, but these limit
you to a fixed position for the transition zone, and a more flexible
approach is probably the square type which slots in to a holder that
in turn screws on to the lens - this allows you to slide the grad filter
up or down to match the horizon in your composition. They can also be
rotated to match a hillside. Even this flexibilty can be rather limited
if your picture includes irregular mountains or a tree against the sky.
RAW mode usually adds a few bits to the range of values that can be
used to describe the intensity of each pixel, typically from 8 bits per
colour (256 levels) with jpeg to 10, 12 or 14 bits (1024, 4096 or 16384
levels) per colour with RAW - however, jpeg probably uses a different
mapping cureve of the numbers to intensities, so you might not see that
much difference. It will however give you more margin for boosting
shadows when using curves adjustments for tone mapping before noise
becomes visible.
These are techniques I use frequently with my Fuji S9500 in RAW mode.
I haven't tried the merging multiple shots taken at different exposures
technique, partly because of the hassle of taking the multiple shots,
but also because many of the examples I've seen look very "false" - the
intensity of the highlights just doesn't look right with the brightness
of the shadows. Doing the tone mapping with a curve from a single
exposure allows a smoother transition of the intensity boost across
different parts of the image.
Some cameras, such as the Fuji S5 Pro, use special sensors with high and
low sensitivity photodiodes to expand the dynamic range.
The Photomatix program has a built-in function to do a tone-mapped
pseudo-HDR from a single RAW file. It does a pretty good job, particularly
on a slightly underexposed image. They have a restricted trial download if
you want to give it a try. Don't remember the URL, but Google should find
it.
"bugbear" <bugbear@trim_papermule.co.uk_trim> wrote in message
news:13dq6e058s5lgc7@corp.supernews.com...
>I have a fairly advanced P&S camera - Canon A630.
>
> I can persuade it to do RAW mode via CHDK
>
> http://scratchpad.wikia.com/wiki/CHDK
>
> I could also use CHDK to take shutter speed
> bracketed shots for HDR usage,
> although this only applies to
> static images, which restricts me somewhat.
>
> I note that it is common to use a grad ND filter
> to reduce burnout in the sky, on "typical"
> landscape shots.
>
> Could I use an ND grad filter, and raw mode,
> and post-apply a correction, so as to
> increase the dynamic range of the image?
>
> This trick would be a pseudo HDR,
> using different exposures (via the grad filter)
> in different parts of the image.
>
> I could then use "normal" HDR tone mapping
> to get my desired final image.
>
> Comments, or hints from people that have already
> done this would be most welcome.
>
> BugBear
On Sep 4, 3:39 am, bugbear <bugbear@trim_papermule.co.uk_trim> wrote:
> I have a fairly advanced P&S camera - Canon A630.
>
> I can persuade it to do RAW mode via CHDK
>
> http://scratchpad.wikia.com/wiki/CHDK
>
> I could also use CHDK to take shutter speed
> bracketed shots for HDR usage,
> although this only applies to
> static images, which restricts me somewhat.
>
> I note that it is common to use a grad ND filter
> to reduce burnout in the sky, on "typical"
> landscape shots.
>
> Could I use an ND grad filter, and raw mode,
> and post-apply a correction, so as to
> increase the dynamic range of the image?
>
> This trick would be a pseudo HDR,
> using different exposures (via the grad filter)
> in different parts of the image.
>
> I could then use "normal" HDR tone mapping
> to get my desired final image.
>
> Comments, or hints from people that have already
> done this would be most welcome.
>
> BugBear
There are a number of techniques to extend dynamic range of an image,
but realize that the graded ND is actually being used to compress
dynamic range of the recorded image. Remember, any hard copy, and
most display techniques, will not have as much dynamic range as a RAW
image of a high contrast scene. The use of high dymaic range work is,
as you say, to map the desired effect on the final image, which,
unless you are using high dynamic range projection systems, must
either compress or clip parts of the image. Which of the two you do
is the "creative judgement".
Cooter wrote:
> The Photomatix program has a built-in function to do a tone-mapped
> pseudo-HDR from a single RAW file. It does a pretty good job, particularly
> on a slightly underexposed image. They have a restricted trial download if
> you want to give it a try. Don't remember the URL, but Google should find
> it.
Don Stauffer in Minnesota wrote:
>
> There are a number of techniques to extend dynamic range of an image,
> but realize that the graded ND is actually being used to compress
> dynamic range of the recorded image.
Indeed; I'm trying to avoid blown hightlights in the sky, whilst
retaining detail in shadows.
> Remember, any hard copy, and
> most display techniques, will not have as much dynamic range as a RAW
> image of a high contrast scene. The use of high dymaic range work is,
> as you say, to map the desired effect on the final image, which,
> unless you are using high dynamic range projection systems, must
> either compress or clip parts of the image. Which of the two you do
> is the "creative judgement".
Yes. The only part of my idea I regard as "original" is that
I intend to "undo" the ND grad, so as to have a "flat"
high dynamic range image. My hope is that I can then tone map
this (as you outline) as if it were a "flat" HDR, and
give no further consideration to the fact that a graduated
filter was used.
Dynamic range, RAW, HDR, Grad ND Filters 
Linear Mode
