[HDR-photo] Following on questions...

[Ferrell McCollough]

John,
20 exposures, Wow! You might want to look into the cutting down on that 
number. The question you need to ask is at what point does the data become 
redundant?

I'm working with realviz and autopano Pro. I'm working on the tutorial for 
Autopano. Very nice software package and makes Hdr really easy.

I could write a whole chapter on Number of Exposures and Bracket Amount and 
as a matter of fact I did. I figured it would be easier to cut and paste so 
here are two small sections from the book. I suggest read the rules for 
choosing histograms then scroll through the images in your camera with the 
histogram displayed. Can you get back with the group as to how many images 
are needed based on the rules. Also, any critique of this is appreciated.

Ferrell





The approach to shooting HDR photography is one of capturing enough dynamic 
range that the HDR merging software has something valuable to work with. 
That means image sets with no gaps due to excessive steps in EV or image 
sets that don't capture the entire dynamic range of the scene. You don't 
want to make giant leaps in EV values but you also don't want to take baby 
steps. The name of the game is overlap of data but not too much overlap that 
it becomes redundant. An analogy would be shooting panoramic image sets, too 
little overlap of images doesn't give the stitching program enough to work 
with and too much overlap doesn't hurt but it does require more images to 
capture the same scene.





........After doing a show-and-tell on histograms I include this concluding 
box:  Choosing Number of Images Based on Histograms

General Rules for Choosing Images Using Histograms


1.        It's always good to keep one of the middle exposures (usually 
0EV). It's valuable just in terms of having a record of the best single shot 
image of the scene. You never know, you may prefer the single shot image 
instead of the HDR image.



2.        Overexposed image. The purpose of the overexposed image is to 
record details in the lower part of the mid-tone and shadow areas of the 
scene. The object is to get enough overexposure to reveal details and reduce 
noise in the shadow areas but no so much over exposure that mid-tones and 
shadows are blown out. The histogram will look like it has been pulled to 
the right. Highlights that were evident in the 0EV image will have gathered 
along the right edge. The shadow and mid-tones will have moved toward the 
right, even spreading wider in a "pulled" appearance. The histogram usually 
develops a "tail" tapering off toward the left edge. The "tail" moving away 
from the left edge is an indication that overexposure is sufficient for the 
scene. An exception would be if you were shooting a scene that contains 
black. The histogram for a scene with black won't taper to a point but will 
have a shoulder effect.



3.        Underexposed image. The underexposed image will record details in 
the highlights of the scene. The histogram for the underexposed image will 
move the highlights toward the middle region. The result will be shadows 
gathered along the left edge, highlights in the middle region and a tail 
stretching to the right hardly touching, if at all, the right edge. The 
"tail" moving away from the right edge is an indication that further 
underexposure is unnecessary. The histogram tail of the underexposed image 
should not show any build up on the right edge, indicating blown pixels. An 
exception would be shooting into the sun or including bright lights in the 
scene. For general photography, some elements are acceptable when left 
overexposed, the disc of the sun, for example.











From: "John Callan" <john at shoutpictures.com>
To: "High Dynamic Range Photography" <hdr-photo at hdr-photography.com>; 
<ustein_outback at yahoo.com>
Sent: Wednesday, February 14, 2007 6:34 AM
Subject: [HDR-photo] Following on questions...


> Dear All
> Just wanted to say thank you to everyone who took the time and effort to
> help me. I haven't replied since my first question because I wanted to
> experiment with your answers. If I may ask a few more please ?
>
> 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 ?
>
> 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 
> ?
>
> Again thank you all.
>
> John  8-)
>
> John Callan.
>
>
>
>
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