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ISO15739 The ISO Definition of Dynamic Range of a Digital Still ...
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ABSTRACT The dynamic range of a digital camera can be simplistically defined as the ratio of the maximum and minimum luminance that a camera can “capture” in a single exposure. But when we try to quantify this property, we find that the establishment of an explicit definition is much more ... ...
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The dynamic range of a digital camera can be simplistically defined as the ratio of
the maximum and minimum luminance that a camera can capture in a single
exposure. But when we try to quantify this property, we find that the
establishment of an explicit definition is much more complicated than it seems on
the surface. International Standard ISO 15739-2003 gives an explicit definition of
dynamic range for a digital still camera and a procedure for determining it. This
article explains the basic concept of dynamic range and discusses some of the
complications in defining it. Then, the definition given by ISO 15739-2003 is
discussed in detail.
The dynamic range of a digital camera is often (very simplistically) defined as the
ratio between the maximum and minimum luminance which, in a single image, can
be successfully captured. Clearly, since many scenes exhibit a large range of
luminance, having a sufficiently-large dynamic range is desirable in attempting to
completely and accurately record such scenes.
What do we really seek in our quest for adequate dynamic range? In general,
from a standpoint of photographic technique, what we are really trying to assure is
that, within the same shot:
Detail carried by small variations in luminance about the highest local average
luminance (that is, the highlight detail) is captured by the camera, and,
simultaneously
Detail carried by small variations in luminance about the lowest local average
luminance (that is, the shadow detail) is captured by the camera
Insufficient dynamic range for the luminance range in the scene will mean that we
cannot simultaneously achieve both of those objectives (again, in a single image).
(We can generally achieve either one by prudent choice of exposure.)
The physical property to which photographic film or the elements of a digital sensor
responds is photometric exposure (symbol: H). At a particular spot on the image,
this is the product of the illuminance on the film or sensor and the time it persists
(, the exposure time).
![](https://csdnimg.cn/release/download_crawler_static/13089773/bg2.jpg)
The ISO Definition of Dynamic Range of a Digital Still Camera Page 2
However, our basic concern is with variations in scene luminance. Because in the
discussion of dynamic range we are ordinarily making comparisons with different
parts of the same image, the aperture used, related factors such as lens
transmission and bellows factor, and the exposure timethe factors that control
the relationship between scene luminance and focal plane photometric exposure
are fixed.
Thus, the ratio between (for example) two values of photometric exposure in an
image will be the same as the ratio between the corresponding two luminance
values in the scene. So for convenience, when discussing the response of a digital
camera sensor (since again it is only ratios that are of importance), we can speak
of the effects of a certain luminance rather than of a certain photometric exposure.
Often, discussions of dynamic range do not revolve around a well-formulated
definition of exactly what ratio is meant. Many times, the definition that is implied
(if not enunciated) has to do with the effect of the quantization of image
luminance at various stages in the processing of the image, starting with the
digitization of the analog voltage by which an individual photodetector reports the
photometric exposure (the illuminance-time product) to which it has been
subjected. Later impacts of quantization occur after we have deduced the specific
color of each pixel in the image ( here of course being a property that
embraces both luminance and chromaticity) from the suite of photodetector data.
For example, if at the stage of the process where we examine the digital image, we
find that the largest luminance of a scene spot that can be digitally represented has
a relative luminance of 4095 units on the applicable digital scale, and the smallest
luminance that receives a non-zero digital representation has a relative luminance of
1 unit, we may be tempted to conclude that the dynamic range of the system (
, an important factor) is 4095
(sometimes stated as 4095:1).
But this doesnt follow our notion of the base luminances for which detail, carried
by luminance variations about the base value, can be perceived.
So perhaps we need to consider the maximum base luminance (the numerator of
our ratio) to be 4094 (allowing detail to be recorded that varies from 4093 units to
4095), and the minimum base luminance (the denominator of our ratio) to be 2
units (allowing detail to be recorded that varies from 1 unit to 3). Now, our numeric
dynamic range would seem to be 2046.5:1. So we see that this little piece of
hair-splitting cuts our assessment of dynamic range about in halfan apparent
degradation of one stop to the photographer.
So perhaps this whole approach to defining dynamic range is ill advised, since the
result we get various so much with how we decide to split the hairs.
![](https://csdnimg.cn/release/download_crawler_static/13089773/bg3.jpg)
The ISO Definition of Dynamic Range of a Digital Still Camera Page 3
Another matter we must consider before settling on a definition of dynamic range
is that of noise, the random variation in the reported luminance of pixels compared
to their actual luminance values. We recognize that in fact the image, for areas
below a certain base luminance, may be so corrupted by noise that we cannot
honestly say that detail carried by small variations of luminance about that base
luminance is captured, at least not in a way that is usable.
Thus, we may eventually wish to think in terms of the dynamic range for the
camera as being defined as the ratio of the highest luminance at which small
luminance differences are recorded to the lowest luminance at which details are
recorded with less than some arbitrarily-established level of noise.
In fact, it is a specific definition along this line that we will discuss in detail here, as
soon as some further philosophical preliminaries are taken care of.
We often speak of the way in which luminance values are captured, when it is
captured and delivered that is really meant. Thus, in discussing the matter of the
dynamic range of a camera, we must be clear as to which form of the image we
will be examining as we seek to determine the dynamic range of the camera in a
laboratory or field test.
Sophisticated photographers may be most interested in the dynamic range as
manifested in the raw
camera output. This output is just a verbatim transcription
of the digitized values of the outputs of all the individual photodetectors across the
camera frame.
In most digital cameras, there is one photodetector for each pixel location in the
image. These are not tristimulus detectors, capable of reporting the color of the
light they receive from some tiny patch on the image. (Note again that color here
means the property that embraces both luminance and chromaticity.)
Rather, these are monochromatic photodetectors that have been given a
response centering on certain wavelengths of the visible spectrum by placing color
filters in front of them, generally of three different kinds, applied in a repeated
pattern across the entire frame (an arrangement often called a color filter array,
or CFA).
Such an array obviously cannot tell us the color of the light falling at any particular
pixel location (either in luminance or chromaticity). But we can deduce the color
1
This is often called the RAW output, although there is no reason for that designation, the word
not being an acronym but merely a metaphor for unprocessed.
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