Doug Kerr
Well-known member
Dynamic range in photography
In the field of photography, the term dynamic range is ordinarily used to express the range (as a ratio) of the maximum to minimum luminance in a scene that can be "successfully captured" in a given shot.
The definitions of "successfully captured" of course can be complicated. In digital photography, often for the maximum we think in terms of the luminance that leads to just short of "saturation" of the sensor. The minimum may be defined in terms of some stated noise performance, or some stated "relative precision" (a metric that is related to the phenomenon of "banding").
This property, dynamic range, is distinct from what we might call the absolute (luminance) range of the camera, which would be the ratio of the maximum to minimum luminances that could be successfully captured, not as part of the scene for a given shot but rather by allowing us to use, for the maximum and minimum, different photographic exposure ISO sensitivity. That range is typically enormous. (We can capture a gigantic luminance at f/22, 1/5000 s, and ISO 50, and a very small one at f/1.4, 2 s, and ISO 8000.)
Said of a scene
Sometimes the term dynamic range is used to speak of the ratio of the maximum to minimum luminance occurring in the part of a scene that would be in the field of view of an actual or hypothetical camera, what is often called its contrast ratio. We recognize that a successful capture of such a scene requires a camera whose dynamic range is at least as great as the contrast ratio of the scene.
I think "dynamic" does not add much - there is not really any meaningful distinction here with some "larger" ratio (unless it might be the ratio between the brightest spot on the scene at high noon vs. the darkest spot at midnight), as we have when we consider measures of the "luminance range" of a camera.
I personally would prefer "contrast ratio", reserving "dynamic range" for the capability of the camera.
About "dynamic"
The descriptor "dynamic" came with the phrase "dynamic range" when it was borrowed from another field, audio recording. There, dynamic range refers to the ratio between the largest and smallest signals (measured at the input to the recording system) that could be "successfully" captured with the recording system gain control at a fixed setting.
Again, the definitions of "successfully captured" can be complicated. Typically for the maximum, it would be the signal amplitude at which the effect of the distortion caused by "saturation" or "clipping" had just reached some stated degree. For the minimum, it was typically defined in terms of some stated noise performance, or some stated impact of certain kinds of distortion that sometimes afflict small signals.
Note that this is not the ultimate range of the recording system. We can successfully record a gigantic signal by greatly reducing the recording gain, and successfully record a tiny signal by greatly increasing the gain. Thus the ultimate range is itself enormous.
Thus the presence of the restrictive clause "with the recording gain control at a fixed setting" in the basic definition. The term "dynamic" fits in with this outlook: it alludes to the range in signal amplitude occurring over the course of a single recording "take" (that is, over the entire piece of music), during which the recording gain is assumed to be kept constant, as distinguished from differences in the amplitude of test signals that might be applied with different gain settings in effect.
Said of the source signal
In recording practice, do we usually speak of the range of maximum to minimum signal amplitudes in an actual (or typical) musical passage as the "dynamic range" of the source signal? ("Recording Sibelius' Second Symphony today, Harvey? Be careful with your gain setup - it has an awesome dynamic range?")
Not usually. If we did, would we be understood unambiguously. Sure.
Color spaces
We will often speak of the dynamic range of a color space. This of course refers to the ratio between the largest and smallest relative luminance that can be successfully captured by the color space. (A normal camera color space does not describe absolute luminance.) As before, we must adopt some particular definition of what the maximum and minimum would mean.
The term "dynamic" can be rationalized here by comparison with our other examples. A certain numerical value of the color of a pixel under a certain color space does not imply an absolute luminance of that spot in the scene. Rather, that relationship depends (among other things) on the photographic exposure and the ISO sensitivity (this being the analog of the recording gain).
Thus, when we speak of the dynamic range of a color space, we mean, "the ratio of the maximum and minimum scene luminance that can be successfully captured assuming a constant photographic exposure and ISO sensitivity of the camera" (which would of course normally mean, "within a given shot").
Said of number systems
Especially in connection with color spaces having a high dynamic range, thus useful for so-called "HDR" (high dynamic range) image handling, we come into contact with certain "advanced" number systems, such as various floating point representations. They are beneficial because they combine a large range with a nearly-constant relative precision while efficiently using a certain number of bits.
Not surprisingly, we are interested in the "range" of these systems, which of course put limit on the range of a color spaces depending on them them. Often it is the "one-sided non-zero relative range" that is of interest. This is the ratio of the largest positive number that can be represented to the smallest non-zero positive number that can be represented.
We sometimes find that called the "dynamic range" of the number system. Is that appropriate?
Well, recall that (based on the history of the term, and consistent with its use in photography), "dynamic" is meant to imply the range with the photographic exposure/recording gain/etc. constant: that is with the scaling factor between the quantity being measured and the "result" the quantity gets held constant.
But in a number system, there is no variable scaling to hold constant. If our number system has a relative range of 1,000,000, there is no way to say that it could actually record two values 100,000,000 part if we "measure them separately, with different settings of the sensitivity control". There is no sensitivity control in a number system. So it only has one kind of range (of any particular flavor.)
So the qualifier "dynamic" has no meaning. Its use in this case is just an attempt to make the discussion "more sophisticated sounding".
So please, let's not speak of the "dynamic range" of a number system.
Multi-shot "HDR" photography.
Often we enlarge the dynamic range of our photographic process by shooting the same scene several times, typically with different photographic exposure, and then combining the images with special software.
Does this increase the dynamic range of the camera? No. For each "shot", that has the same value.
But we have increased the dynamic range of the entire photographic process (in terms of its ability to "successfully" capture a certain range of scene luminance).
What if we do not record the result of the combining in a high-dynamic range color space, but rather use tonal compression of some sort and then record the result in a color space of more modest dynamic range. Have we then still increased the dynamic range of the entire photographic process?
Yes, because that dynamic range is defined as the ability (in this case of the entire process) to successfully capture a certain range of maximum to minimum scene luminance, and that is in fact enlarged.
Now, does the delivered image in that case have the same contrast ratio as the scene itself. No. What about its dynamic range? I'd prefer not to use that term there, but no.
Best regards,
Doug
In the field of photography, the term dynamic range is ordinarily used to express the range (as a ratio) of the maximum to minimum luminance in a scene that can be "successfully captured" in a given shot.
The definitions of "successfully captured" of course can be complicated. In digital photography, often for the maximum we think in terms of the luminance that leads to just short of "saturation" of the sensor. The minimum may be defined in terms of some stated noise performance, or some stated "relative precision" (a metric that is related to the phenomenon of "banding").
This property, dynamic range, is distinct from what we might call the absolute (luminance) range of the camera, which would be the ratio of the maximum to minimum luminances that could be successfully captured, not as part of the scene for a given shot but rather by allowing us to use, for the maximum and minimum, different photographic exposure ISO sensitivity. That range is typically enormous. (We can capture a gigantic luminance at f/22, 1/5000 s, and ISO 50, and a very small one at f/1.4, 2 s, and ISO 8000.)
Said of a scene
Sometimes the term dynamic range is used to speak of the ratio of the maximum to minimum luminance occurring in the part of a scene that would be in the field of view of an actual or hypothetical camera, what is often called its contrast ratio. We recognize that a successful capture of such a scene requires a camera whose dynamic range is at least as great as the contrast ratio of the scene.
I think "dynamic" does not add much - there is not really any meaningful distinction here with some "larger" ratio (unless it might be the ratio between the brightest spot on the scene at high noon vs. the darkest spot at midnight), as we have when we consider measures of the "luminance range" of a camera.
I personally would prefer "contrast ratio", reserving "dynamic range" for the capability of the camera.
About "dynamic"
The descriptor "dynamic" came with the phrase "dynamic range" when it was borrowed from another field, audio recording. There, dynamic range refers to the ratio between the largest and smallest signals (measured at the input to the recording system) that could be "successfully" captured with the recording system gain control at a fixed setting.
Again, the definitions of "successfully captured" can be complicated. Typically for the maximum, it would be the signal amplitude at which the effect of the distortion caused by "saturation" or "clipping" had just reached some stated degree. For the minimum, it was typically defined in terms of some stated noise performance, or some stated impact of certain kinds of distortion that sometimes afflict small signals.
Note that this is not the ultimate range of the recording system. We can successfully record a gigantic signal by greatly reducing the recording gain, and successfully record a tiny signal by greatly increasing the gain. Thus the ultimate range is itself enormous.
Thus the presence of the restrictive clause "with the recording gain control at a fixed setting" in the basic definition. The term "dynamic" fits in with this outlook: it alludes to the range in signal amplitude occurring over the course of a single recording "take" (that is, over the entire piece of music), during which the recording gain is assumed to be kept constant, as distinguished from differences in the amplitude of test signals that might be applied with different gain settings in effect.
Said of the source signal
In recording practice, do we usually speak of the range of maximum to minimum signal amplitudes in an actual (or typical) musical passage as the "dynamic range" of the source signal? ("Recording Sibelius' Second Symphony today, Harvey? Be careful with your gain setup - it has an awesome dynamic range?")
Not usually. If we did, would we be understood unambiguously. Sure.
I have wonderful recordings, from the 1950's, where Harvey did not hark to that advice and, in the middle of an extended crescendo, discovered from watching the VU meter that saturation was in his future, and awkwardly reduced the gain suddenly at a breath pause in the middle of a passage.
Color spaces
We will often speak of the dynamic range of a color space. This of course refers to the ratio between the largest and smallest relative luminance that can be successfully captured by the color space. (A normal camera color space does not describe absolute luminance.) As before, we must adopt some particular definition of what the maximum and minimum would mean.
The term "dynamic" can be rationalized here by comparison with our other examples. A certain numerical value of the color of a pixel under a certain color space does not imply an absolute luminance of that spot in the scene. Rather, that relationship depends (among other things) on the photographic exposure and the ISO sensitivity (this being the analog of the recording gain).
Thus, when we speak of the dynamic range of a color space, we mean, "the ratio of the maximum and minimum scene luminance that can be successfully captured assuming a constant photographic exposure and ISO sensitivity of the camera" (which would of course normally mean, "within a given shot").
Said of number systems
Especially in connection with color spaces having a high dynamic range, thus useful for so-called "HDR" (high dynamic range) image handling, we come into contact with certain "advanced" number systems, such as various floating point representations. They are beneficial because they combine a large range with a nearly-constant relative precision while efficiently using a certain number of bits.
Not surprisingly, we are interested in the "range" of these systems, which of course put limit on the range of a color spaces depending on them them. Often it is the "one-sided non-zero relative range" that is of interest. This is the ratio of the largest positive number that can be represented to the smallest non-zero positive number that can be represented.
We sometimes find that called the "dynamic range" of the number system. Is that appropriate?
Well, recall that (based on the history of the term, and consistent with its use in photography), "dynamic" is meant to imply the range with the photographic exposure/recording gain/etc. constant: that is with the scaling factor between the quantity being measured and the "result" the quantity gets held constant.
But in a number system, there is no variable scaling to hold constant. If our number system has a relative range of 1,000,000, there is no way to say that it could actually record two values 100,000,000 part if we "measure them separately, with different settings of the sensitivity control". There is no sensitivity control in a number system. So it only has one kind of range (of any particular flavor.)
So the qualifier "dynamic" has no meaning. Its use in this case is just an attempt to make the discussion "more sophisticated sounding".
So please, let's not speak of the "dynamic range" of a number system.
Multi-shot "HDR" photography.
Often we enlarge the dynamic range of our photographic process by shooting the same scene several times, typically with different photographic exposure, and then combining the images with special software.
Does this increase the dynamic range of the camera? No. For each "shot", that has the same value.
But we have increased the dynamic range of the entire photographic process (in terms of its ability to "successfully" capture a certain range of scene luminance).
What if we do not record the result of the combining in a high-dynamic range color space, but rather use tonal compression of some sort and then record the result in a color space of more modest dynamic range. Have we then still increased the dynamic range of the entire photographic process?
Yes, because that dynamic range is defined as the ability (in this case of the entire process) to successfully capture a certain range of maximum to minimum scene luminance, and that is in fact enlarged.
Now, does the delivered image in that case have the same contrast ratio as the scene itself. No. What about its dynamic range? I'd prefer not to use that term there, but no.
Best regards,
Doug
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