[HDR-photo] Following on questions...

[Geraldine Joffre]

On Wed, 14 Feb 2007 10:42:20 -0700, Royce Howland wrote
> 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. 
I believe there are still benefits when using cameras that are prone to noise,
or if you need to shoot at hight ISO. But then the advantages gained have to
be balanced by the disadvantages you mentioned regarding the higher risk of
frame-to-frame alignment problems and introducing ghosts. 

> 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. 
More samples will still help improve the quality of the HDR image, thanks to a
more accurate calculation of the HDR values and to the noise reduction
benefits produced by averaging.

> 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. 
The usual approach is to calculate a weighted average of the pixel values for
each frame after linearization and division by the frame exposure.  The
weighting is a function of the pixel value of the frame and intended to favor
the best exposed pixels.  One example of such weighting function is a bell
curve centered on the middle of the tonal range reproduced.

> 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. 
Yes. Based on our tests, using images separated by 3 EV for the HDR image
result in tone mapped images that are less "smooth" than when the source
images are separated by 2 EV.

Geraldine Joffre