[HDR-photo] Following on questions...

[Geraldine Joffre]

On Wed, 14 Feb 2007 12:34:59 +0100, John Callan wrote
> 1. I took some night time shots of a local town with street cafes. 
> In all I took 20 different exposures of each view. On running them 
> through photomatix I found that the composite containing the 20 
> exposures looked much better then the others containing various 
> amounts ranging from 3 to 12. I have therefore concluded that it's 
> best to shoot and combine a large number of exposures. Would anyone 
> disagree with this and why ?
I would not disagree.

While I agree there is no fast & hard rule regarding the exposure spacing, I
believe a rule does exist regarding the exposure range. This rule is that the
differently exposed photos should properly cover the dynamic range of the
scene, or at least the dynamic range you want to capture. Once you have
determined the lowest and highest exposure values you need for your scene, the
number of exposures will then depend of the choice of the EV spacing.

With most outdoors scenes, 3 exposures spaced by two EV will work fine.
However, this should not be taken as a rule for other types of scene. If you
are an architecture photographers and are taking interiors with a view outside
the window, you are likely to get disappointing tone mapped results with just
3 exposures. You will usually need at least 5 exposures with a two EV-spacing,
and 9 exposures with a one EV-spacing.

In the case you are describing of a night shot including neon signs and street
lights, the dynamic range of such scene can be much higher than a typical
daylight landscape scene that excludes the sun. You will need more than 3
exposures to capture everything correctly. Then, because it is a night shot,
it will be better to select a relatively small EV spacing (one f-stop or even
less) in order to smooth out noise in the dark parts of the scene, and this
will increase the number of exposures needed too.

> 2. I use Realviz stitcher to stitch my panoramics. They advise me to 
> HDR then stitch. Is this software the best performing for stitching 
> HDR images ?
To my knowledge the blending of Stitcher is quite good at smoothing difference
in brightness between the tone mapped frames. However, it is still possible to
use Stitcher with the stitch-then-HDR workflow (i.e. stitch one pano per
exposure, then merge the differently exposed panos into HDR) which is the
recommended workflow if you are using a tone mapping method that works locally
(e.g. Details Enhancer in Photomatix). The details on how to do that with
Stitcher are here
http://www.hdrsoft.com/support/faq_photomatix.html#stitcher

Geraldine Joffre