1/17/14 11:04 AMUnderstanding Dynamic Range in Digital Photography

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DYNAMIC RANGE IN DIGITALPHOTOGRAPHYDynamic range in photography describes the ratio between the maximum and minimum measurable lightintensities (white and black, respectively). In the real world, one never encounters true white or black —only varying degrees of light source intensity and subject reflectivity. Therefore the concept of dynamicrange becomes more complicated, and depends on whether you are describing a capture device (such as acamera or scanner), a display device (such as a print or computer display), or the subject itself.

Just as with color management, each device within the above imaging chain has their own dynamic range.In prints and computer displays, nothing can become brighter than paper white or a maximum intensitypixel, respectively. In fact, another device not shown above is our eyes, which also have their own dynamic

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range. Translating image information between devices may therefore affect how that image is reproduced.The concept of dynamic range is therefore useful for relative comparisons between the actual scene, yourcamera, and the image on your screen or in the final print.

INFLUENCE OF LIGHT: ILLUMINANCE & REFLECTIVITYLight intensity can be described in terms of incident and reflected light; both contribute to the dynamicrange of a scene (see tutorial on "camera metering and exposure").

Strong Reflections

Uneven Incident Light

Scenes with high variation in reflectivity, such as those containing black objects in addition to strongreflections, may actually have a greater dynamic range than scenes with large incident light variation.Photography under either scenario can easily exceed the dynamic range of your camera — particularly if theexposure is not spot on.

Accurate measurement of light intensity, or luminance, is therefore critical when assessing dynamic range.Here we use the term illuminance to specify only incident light. Both illuminance and luminance aretypically measured in candelas per square meter (cd/m2). Approximate values for commonly encounteredlight sources are shown below.

Here we see the vast variation possible for incident light, since the above diagram is scaled to powers of ten.If a scene were unevenly illuminated by both direct and obstructed sunlight, this alone can greatly increase ascene's dynamic range (as apparent from the canyon sunset example with a partially-lit cliff face).

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DIGITAL CAMERASAlthough the meaning of dynamic range for a real-world scene is simply the ratio between lightest anddarkest regions (contrast ratio), its definition becomes more complicated when describing measurementdevices such as digital cameras and scanners. Recall from the tutorial on digital camera sensors that light ismeasured at each pixel in a cavity or well (photosite). Each photosite's size, in addition to how its contentsare measured, determine a digital camera's dynamic range.

Black Level(Limited by Noise)

White Level(Saturated Photosite)

Darker White Level(Low Capacity Photosite)

Photosites can be thought of as buckets which hold photons as if they were water. Therefore, if the bucketbecomes too full, it will overflow. A photosite which overflows is said to have become saturated, and istherefore unable to discern between additional incoming photons — thereby defining the camera's whitelevel. For an ideal camera, its contrast ratio would therefore be just the number of photons it could containwithin each photosite, divided by the darkest measurable light intensity (one photon). If each held 1000photons, then the contrast ratio would be 1000:1. Since larger photosites can contain a greater range ofphotons, dynamic range is generally higher for digital SLR cameras compared to compact cameras(due to larger pixel sizes).

Technical Note: In some digital cameras, there is an extended low ISO setting which produces less noise,but also decreases dynamic range. This is because the setting in effect overexposes the image by a full f-stop, but then later truncates the highlights — thereby increasing the light signal. An example of this ismany of the Canon cameras, which have an ISO-50 speed below the ordinary ISO-100.

In reality, consumer cameras cannot count individual photons. Dynamic range is therefore limited by thedarkest tone where texture can no longer be discerned; we call this the black level. The black level islimited by how accurately each photosite can be measured, and is therefore limited in darkness by imagenoise. Therefore, dynamic range generally increases for lower ISO speeds and cameras with lessmeasurement noise.

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Techncal Note: Even if a photosite could count individual photons, it would still be limited by photonnoise. Photon noise is created by the statistical variation in arrival of photons, and therefore represents atheoretical minimum for noise. Total noise represents the sum of photon noise and read-out noise.

Overall, the dynamic range of a digital camera can therefore be described as the ratio of maximumlight intensity measurable (at pixel saturation), to minimum light intensity measurable (above read-out noise). The most commonly used unit for measuring dynamic range in digital cameras is the f-stop,which describes total light range by powers of 2. A contrast ratio of 1024:1 could therefore also bedescribed as having a dynamic range of 10 f-stops (since 210 = 1024). Depending on the application, eachunit f-stop may also be described as a "zone" or "eV."

SCANNERSScanners are subject to the same saturation:noise criterion as for dynamic range in digital cameras, except itis instead described in terms of density (D). This is useful because it is conceptually similar to howpigments create tones in printed media, as shown below.

Low Reflectance(High Density)

High Reflectance(Low Density)

High Pigment Density(Darker Tone)

Low Pigment Density(Lighter Tone)

The overall dynamic range in terms of density is therefore the maximum pigment density (Dmax), minus theminimum pigment density (Dmin). Unlike powers of 2 for f-stops, density is measured using powers of 10(just as the Richter scale for earthquakes). A density of 3.0 therefore represents a contrast ratio of 1000:1(since 103.0 = 1000).

Dynamic Rangeof Original

Dynamic Rangeof Scanner

Instead of listing total density (D), scanner manufacturer's typically list just the Dmax value, since Dmax -Dmin is approximately equal to Dmax. This is because unlike with digital cameras, a scanner has full controlover it's light source, ensuring that minimal photosite saturation occurs.

For high pigment density, the same noise constraints apply to scanners as digital cameras (since they bothuse an array of photosites for measurement). Therefore the measurable Dmax is also determined by the noise

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present during read-out of the light signal.

COMPARISONDynamic range varies so greatly that it is commonly measured on a logarithmic scale, similar to how vastlydifferent earthquake intensities are all measured on the same Richter scale. Here we show the maximummeasurable (or reproducible) dynamic range for several devices in terms any preferred measure (f-stops,density and contrast ratio). Move your mouse over each of the options below to compare these.

Select Measure for Dynamic Range:f-stops Density Contrast Ratio

Select Types to Display Above:Printed Media Scanners Digital Cameras Display Devices

Note the huge discrepancy between reproducible dynamic range in prints, and that measurable by scannersand digital cameras. For a comparison with real-world dynamic range in a scene, these vary fromapproximately 3 f-stops for a cloudy day with nearly even reflectivity, to 12+ f-stops for a sunny day withhighly uneven reflectivity.

Care should be taken when interpreting the above numbers; real-world dynamic range is a strong functionof ambient light for prints and display devices. Prints not viewed under adequate light may not give theirfull dynamic range, while display devices require near complete darkness to realize their full potential —especially for plasma displays. Finally, these values are rough approximations only; actual values depend onage of device, model generation, price range, etc.

Be warned that contrast ratios for display devices are often greatly exaggerated, as there is nomanufacturer standard for listing these. Contrast ratios in excess of 500:1 are often only the result of a verydark black point, instead of a brighter white point. For this reason attention should be paid to both contrastratio and luminosity. High contrast ratios (without a correspondingly higher luminosity) can be completelynegated by even ambient candle light.

THE HUMAN EYEThe human eye can actually perceive a greater dynamic range than is ordinarily possible with a camera. Ifwe were to consider situations where our pupil opens and closes for varying light, our eyes can see over arange of nearly 24 f-stops.

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On the other hand, for accurate comparisons with a single photo (at constant aperture, shutter and ISO), wecan only consider the instantaneous dynamic range (where our pupil opening is unchanged). This would besimilar to looking at one region within a scene, letting our eyes adjust, and not looking anywhere else. Forthis scenario there is much disagreement, because our eye's sensitivity and dynamic range actually changedepending on brightness and contrast. Most estimate anywhere from 10-14 f-stops.

The problem with these numbers is that our eyes are extremely adaptable. For situations of extreme low-light star viewing (where our eyes have adjusted to use rod cells for night vision), our eyes approach evenhigher instantaneous dynamic ranges (see tutorial on "Color Perception of the Human Eye").

BIT DEPTH & MEASURING DYNAMIC RANGEEven if one's digital camera could capture a vast dynamic range, the precision at which light measurementsare translated into digital values may limit usable dynamic range. The workhorse which translates thesecontinuous measurements into discrete numerical values is called the analog to digital (A/D)converter. The accuracy of an A/D converter can be described in terms of bits of precision, similar to bitdepth in digital images, although care should be taken that these concepts are not used interchangeably. TheA/D converter is what creates values for the digital camera's RAW file format.

Bit Precisionof Analog/Digital

ConverterContrast Ratio

Dynamic Range

f-stops Density

8 256:1 8 2.410 1024:1 10 3.012 4096:1 12 3.614 16384:1 14 4.216 65536:1 16 4.8

Note: Above values are for A/D converter precision only,and should not be used to interpret results for 8 and 16-bit image files.Furthermore, values shown are a theoretical maximum, assuming noise is not limiting.Additionally, this applies only to linear A/D converters; a non-linear A/D converter's bit precision does notnecessarily correlate with dynamic range.

As an example, 10-bits of tonal precision translates into a possible brightness range of 0-1023 (since 210 =1024 levels). Assuming that each A/D converter number is proportional to actual image brightness

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(meaning twice the pixel value represents twice the brightness), 10-bits of precision can only encode acontrast ratio of 1024:1.

Most digital cameras use a 10 to 14-bit A/D converter, and so their theoretical maximum dynamic range is10-14 stops. However, this high bit depth only helps minimize image posterization since total dynamicrange is usually limited by noise levels. Similar to how a high bit depth image does not necessarily meanthat image contains more colors, if a digital camera has a high precision A/D converter it does notnecessarily mean it can record a greater dynamic range. In practice, the dynamic range of a digitalcamera does not even approach the A/D converter's theoretical maximum; 8-12 stops is generally allone can expect from the camera.

INFLUENCE OF IMAGE TYPE & TONAL CURVECan digital image files actually record the full dynamic range of high-end devices? There seems to be muchconfusion on the internet about the relevance of image bit depth on recordable dynamic range.

We first need to distinguish between whether we are speaking of recordable dynamic range, or displayabledynamic range. Even an ordinary 8-bit JPEG image file can conceivably record an infinite dynamic range— assuming that the right tonal curve is applied during RAW conversion (see tutorial on curves, undermotivation: dynamic range), and that the A/D converter has the required bit precision. The problem lies inthe usability of this dynamic range; if too few bits are spread over too great of a tonal range, then this canlead to image posterization.

On the other hand, displayable dynamic range depends on the gamma correction or tonal curve implied bythe image file, or used by the video card and display device. Using a gamma of 2.2 (standard for PC's), itwould be theoretically possible to encode a dynamic range of nearly 18 f-stops (see tutorial on gammacorrection, to be added). Again though, this would suffer from severe posterization. The only currentstandard solution for encoding a nearly infinite dynamic range (with no visible posterization) is to use highdynamic range (HDR) image files in Photoshop (or other supporting program).

Want to learn more? Discuss this and other articles in our digital photography forums.

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