The collective thought of the Studio Forum on the subject of... "Extending the Dynamic Range"

Maximum 4½ -stop range:
No flat lit subject has a range greater than a 4½ -stop range. This is well within the approximate 6-stop range of our camera's sensor. Let me explain this 4½ -stop range.
There is no object that absorbs 100% of the light falling on it. A common photographic material at this darkest range is black velvet... Black velvet reflects about 4% of the light.
There is no object that reflects 100% of the light falling on it. One of the brightest materials is magnesium oxide which reflects about 90% of the light.
Given that each doubling of the light equals a 1-stop change then there can only be 4½-stops for any given flatly lighted range:

4% to 8%	8% to 16%	16% to 32%	32% to 64%	64% to 96%
1st stop	2nd stop	3rd stop	4th stop	another ½-stop

That said, a building that is flatly lit, whether in sunlight or shade, can have no greater than a 4½-stops range… although the camera's exposure for these to lighting situations would change greatly.

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Multiple 4½ -stop ranges:
However, if the building was lit by Sun, it would cast a shadow which would have its own maximum 4½ -stop range.
If this brightest areas of the shadow area picked up its 4½ -stop tonal range exactly where the darkest areas of the sun lit building left off, then the photographic scene would have a 9-stop range - or about 1½ X the range of our camera's 6-stop sensor.
If the brightest areas of the shadow area does not pick up where the darkest areas of the sun lit building left off - let's say a 3-stop gap - then the dynamic range of the scene would be more like 12-stops or 2x greater than our camera's 6-stop sensor.
Scene brightness differences between the brightest and darkest flat light tonal range areas can be 4 to 6-stop difference making the dynamic range of the scene between 13 to 15-stops! We are talking about simple scenes... a building and its shadow area or a building and its interior. However there may be a second building or a car interior within the photo giving a scene multiple overlapping 4½ -stop flat light tonal range areas. Enter HDR or Extending the Dynamic Range...

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Extending the Dynamic Range:
If we could make multiple exposures capturing each brightness area's 4½ -stop range and use software that could combine these multiple exposures of various brightness ranges - we could extend the dynamic range beyond what the camera sensor can capture.
Enter the plethora of [all the HDR/XDR]software that can extend the dynamic range of the scene: Photomatix, ReDynaMix, Enfuse, Photoshop, etc. It may be beyond the scope of this paper to discuss the hows and whys of these software… but the intent of all of them is to capture these great ranges of brightness areas and incorporate them into one Extended Dynamic Range (XDR) file.
One of the more common ways to combine these multiple exposures from the average digital 12 bit (4096 shades of brightness) cameras sensor is to put them into a high-definition range (HDR) 32-bit file containing 65,530 shades of brightness. We would then try to reshape the tonal curve to present the image as normal as possible - this is called "tone mapping". I find "Tone-mapping" challenging, subject dependent, laden with artifacts such as haloing, color shifts along the tonal range (a.k.a. "the grunge look"), etc.
A second, less common way, would be to blend selected "properly exposed areas" of the various multiple exposures into one "normally" exposed file- let's call this "blending". In either case, the software attempts at combining multiple exposures into one extended range file is very subject dependent -- meaning when the results are good... they are great.

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The Big Question… What is Our Goal in Extending the Dynamic Range?
For this paper I will use Extended Dynamic Range XDR when talking about the concept and HDR when talking about the process. What is the purpose for shooting high or Extended Dynamic Range images…
- to create a "grunge" or stylized effect or
- to create a representation of the scene how our eyes more normally saw it?
The answer here is personal, so personal speaking; I want to create a representation of the scene I viewed and then process the file in familiar software (Photoshop/Lightroom) to enhance the additional ranges of brightness in the file to create my expression. I would suspect this is the goal for most of us.
The learning curve for "tone-mapping" is steep; the processing is different for each image; and the process is time-consuming. "Blending" does not require critical tone mapping, provides a more predictable results and a faster process... that is why I prefer the "blending" technique for Extending Dynamic Range images.

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Bit This - Extending the Dynamic Range:
If a digital camera's 12-bit sensor can record 4,096 shades of brightness in a 6-stop range; and a 32-bit HDR file can hold 65,536 shades of brightness; in a 32-bit HDR file there would have an additional 30,000 shades or 4-stops of brightness. "Blending" images extracts the best detail areas and fuses them into a 16 or 8 bit file needing to tone-mapping.

12-bit Camera sensor:
				1st Stop	2nd Stop	3rd Stop	4th Stop	5th Stop	6th Stop
--	--	--	--	2048	1024	512	256	128	64
32-bit file:
+1	+2	+3	+4	1st Stop	2nd Stop	3rd Stop	4th Stop	5th Stop	6th Stop
32,768	16,384	8,192	4,096	2048	1024	512	256	128	64