[HDR-photo] Following on questions...

[Jack Howard]

My general rule, when manual bracketing, is to shoot so the absolute
hottest image elements are about 2/3 stop underexposed to when the
darkest shadows are about 2/3 stop overexposed at about 1-stop
increments...and experiment like crazy!

It's also sometimes helpful to downsample a copy of the series to
screen-sized for rough drafting, to speed processing, when using this
method.

Jack

On 2/14/07, Royce Howland <royce at cospring.com> wrote:
> John Callan wrote:
> > [...]
> > Around the 12th exposure I was beginning to get the desired effect and so
> > worked on from there.
> >
> > Am I doing something horrendously wrong ?
>
> Like Jack Howard stated, there are no hard & fast rules. However 12 - 20
> exposures sounds somewhat extreme to me. I infer from that number that
> your exposures are fairly close to each other, i.e. at 1 EV or less of
> separation.
>
> Doing outdoor shots & stitched panoramics, I get a lot of successful
> images with 3 shots taken at -2, 0 and +2 EV. Other shots require 5 or 6
> exposures, and I have never shot more than 9. In general I use a 2 EV
> step between exposures. How many shots are needed is a function of the
> scene's total dynamic range, how much of that DR must be captured
> (rarely need to see detail on the surface of the sun for example), and
> how widely spaced the exposures are.
>
> In general, fewer shots are easier to post-process, especially to reduce
> the risk of frame-to-frame alignment problems that rob image sharpness
> or introduce ghosts that must be processed out (if you don't want them).
> Most folks I know shoot their sequences at 1.67 or 2 EV apart to balance
> quality of HDR sampling with a reasonable number of images in the sequence.
>
> In my own testing (which I admit is more pragmatic than it is rigorous),
> I have not yet found any compelling benefit to shooting at 1 EV rather
> than 2 EV of separation. This is because each exposure has a "sweet
> spot" several stops wide that contains good contrast & detail; these
> sweet spots are on the upper part of the histogram where the majority of
> luminance levels are recorded. As long as those sweet spots reasonably
> overlap from one exposure to the next, overlapping them even more does
> not buy you any real benefit. (The analogy in another response was
> overlapping shots for stitching.)
>
> The trade-off here is giving the HDR merge algorithm enough luminance
> levels to sample so it can pull in good detail across the entire tonal
> range. Once enough is enough, throwing even more sampling points at the
> HDR tool doesn't help anything. At a 2 EV step I would bet that most
> pixels in "normal" scenes (not heavily biased towards the shadows or
> highlights) have 2 - 3 clean exposures to sample during the merge, at a
> level of detail that is useful enough. But how the HDR tools elect where
> to sample, I'm not sure. I guess the simplest approach would be to pick
> the exposure where the region to be sampled is shifted the furthest
> right on the histogram without being clipped; this would provide the
> largest number of luminance levels to pick from.
>
> Going a bit more than a 2 EV step may help by cutting the number of
> exposures required, although at the risk of pushing the merge sampling
> further apart. But since most cameras have auto-bracket functions that
> are limited to a maximum of 2 EV steps, using more than that means
> touching the camera for every shot to manually adjust exposure. Again
> this risks introduction of alignment errors. Plus it takes a few more
> seconds per shot, which increases the odds of something more significant
> changing in the scene before the sequence can be finished. So I normally
> prefer to use 2 EV, shoot an auto-bracket of 3 shots, and manually
> adjust the exposure to take successive sets of 3 shots if I need more DR
> capture.
>
> This is my interpretation as a pragmatic-oriented guy just trying to get
> the best out of the tools with minimum effort. :) I'd be happy to see
> comments on this from somebody that knows the insides of HDR sampling...
>
> Royce Howland
>
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-- 
Jack Howard
732/682-7628
www.sportsshooter.com/jackhoward