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- HDR: HIGH DYNAMIC RANGE

PHOTOGRAPHY -
High dynamic range (HDR) images enable photographers to record a greater range of tonal
detail than a given camera could capture in a single photo. This opens up a whole new set of
lighting possibilities which one might have previously avoided—for purely technical reasons.
The new "merge to HDR" feature of Photoshop CS2 allows the photographer to combine a
series of bracketed exposures into a single image which encompasses the tonal detail of the
entire series. There is no free lunch however; trying to broaden the tonal range will
inevitably come at the expense of decreased contrast in some tones. Learning to use the
merge to HDR feature in Photoshop CS2 can help you make the most of your dynamic range
under tricky lighting—while still balancing this trade-off with contrast.

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MOTIVATION: THE DYNAMIC RANGE DILEMMA

As digital sensors attain progressively higher resolutions, and thereby successively smaller
pixel sizes, the one quality of an image which does not benefit is its dynamic range. This is
particularly apparent in compact cameras with resolutions near 8 megapixels, as these are
more susceptible than ever to blown highlights or noisy shadow detail. Further, some scenes
simply contain a greater brightness range than can be captured by current digital cameras--
of any type.

The "bright side" is that nearly any camera can actually capture a vast dynamic range-- just
not in a single photo. By varying the shutter speed alone, most digital cameras can change
how much light they let in by a factor of 50,000 or more. High dynamic range imaging
attempts to utilize this characteristic by creating images composed of multiple exposures,
which can far surpass the dynamic range of a single exposure.

WHEN TO USE HDR IMAGES

I would suggest only using HDR images when the scene's brightness distribution can no
longer be easily blended using a graduated neutral density (GND) filter. This is because GND
filters extend dynamic range while still maintaining local contrast. Scenes which are ideally
suited for GND filters are those with simple lighting geometries, such as the linear blend from
dark to light encountered commonly in landscape photography (corresponding to the
relatively dark land transitioning into bright sky).

GND Filter Final Result

In contrast, a scene whose brightness distribution is no longer easily blended using a GND
filter is the doorway scene shown below
filter is the doorway scene shown below.

Brightness Distribution Underexposure Overexposure

We note that the above scene contains roughly three tonal regions with abrupt transitions at
their edges-- therefore requiring a custom-made GND filter. If we were to look at this in
person, we would be able to discern detail both inside and outside the doorway, because our
eyes would adjust to changing brightness. The goal of HDR use in this article is to better
approximate what we would see with our own eyes through the use of a technique called
tonal mapping.

INNER WORKINGS OF AN HDR FILE

Photoshop creates an HDR file by using the EXIF information from each of your bracketed
images to determine their shutter speed, aperture and ISO settings. It then uses this
information to assess how much light came from each image region. Since this light may
vary greatly in its intensity, Photoshop creates the HDR file using 32-bits to describe each
color channel (as opposed to the usual 16 or 8-bits, as discussed in the tutorial on
"Understanding Bit Depth"). The real benefit is that HDR files use these extra bits to create a
relatively open-ended brightness scale, which can adjust to fit the needs of your image. The
important distinction is that these extra bits are used differently than the extra bits in 16-bit
images, which instead just define tones more precisely (see tutorials on the "RAW File
Format" and "Posterization"). We refer to the usual 8 and 16-bit files as being low dynamic
range (LDR) images, relatively speaking.

The 32-bit HDR file format describes a greater dynamic range by using its bits to
specify floating point numbers, also referred to as exponential notation. A floating point
number is represented by a fractional numerical value raised to some power of 10, such as
5.467x103, as opposed to the usual 0-255 (for 8-bit) or 0-65535 (for 16-bit) integer color
specifications. This way, an image file can specify a brightness of 4,300,000,000 simply as
4.3x109, which would be too large even with 32-bit integers.

We see that the floating point notation certainly looks neater and more concise, but how does
this help a computer? Why not just keep adding more bits to specify successively larger
numbers, and therefore a larger dynamic range? Recall that for ordinary LDR files, far more
bits are used to distinguish lighter tones than darker tones (from the tutorial on gamma
correction, tonal levels and exposure - to be added). As a result, as more bits are added, an
exponentially greater fraction of these bits are used to specify color more precisely, instead of
extending dynamic range.
 Representation of How Bits Are Allocated for Increasing Brightness
Note: Above representation is qualitative, and depends on other factors such as screen bit depth, monitor
gamma, etc. The more closely spaced bits for brighter values is a result of the fact that ordinary 8 and 16-bit
JPEG files are gamma-encoded, which can actually help increase dynamic range for low-bit files; gamma-
encoding just becomes more and more inefficient as the bit depth increases.

HDR files get around this LDR dilemma of diminishing returns by using floating point numbers
which are proportional to the actual brightness values of the subject matter (gamma equals
one, or linear). This ensures that bits are equally spaced throughout the dynamic range, and
not just concentrated in the brighter tones-- allowing for greater bit efficiency. Further, the
use of floating point numbers ensure that all tones are recorded with the same relative
precision, since the base (mantissa) in numbers such as 2.576x103 and 8.924x109 each have
the same number of significant figures (four), even though the second number is more than a
million times larger.

Note: just as how using high bit depth images do not necessarily mean your image contains more color, a high
dynamic range file does not guarantee greater dynamic range unless this is also present in the actual subject
matter.

All of these extra bits provided by the HDR format are great, and effectively allow for a nearly
infinite brightness range to be described. The problem is that your computer display (or the
final photographic print) can only show a fixed brightness scale. This tutorial therefore
focuses on how to create and convert HDR files into an ordinary 8 or 16-bit image, which can
be displayed on a monitor, or will look great as a photographic print. This process is also
commonly referred to as tonal mapping.

IN-FIELD PREPARATION
Since creating a HDR image requires capturing a series of identically-positioned exposures, a
sturdy tripod is essential. Photoshop has a feature which attempts to align the images when
the camera may have moved between shots, however best results are achieved when this is
not relied upon.

Make sure to take at least three exposures, although five or more is recommended for
optimum accuracy. More exposures allow the HDR algorithm to better approximate how your
camera translates light into digital values (a.k.a. the digital sensor's response curve)--
creating a more even tonal distribution. The doorway example is best-suited with several
intermediate exposures, in addition to the two shown previously.

Reference -1 Stops -2 Stops -3 Stops

It is essential that the darkest of these exposures includes no blown highlights in areas where
you want to capture detail. The brightest exposure should show the darkest regions of the
image with enough brightness that they are relatively noise-free and clearly visible. Each
exposure should be separated by one to two stops, and these are ideally set by varying the
shutter speed (as opposed to aperture or ISO speed). Recall that each "stop" refers to a
doubling (+1 stop) or halving (-1 stop) of the light captured from an exposure.

We also note another disadvantage of HDR images: they require relatively static subject
matter, due to the necessity of several separate exposures. Our previous ocean sunset
example would therefore not be well-suited for the HDR technique, as the waves would have
moved significantly between each exposure.
 CREATING A 32-BIT HDR FILE IN PHOTOSHOP CS2
Here we use Adobe Photoshop CS2 to convert the sequence of exposures into a single image,
which uses tonal mapping to approximate what we would see with our eye. Before tonal
mapping can be performed, we first need to combine all exposures into a single 32-bit HDR
file.

Open the HDR tool (File>Automate>Merge to HDR…), and load all photographs in the
exposure sequence; for this example it would be the four images shown in the previous
section. If your images were not taken on a stable tripod, this step may require checking
"Attempt to Automatically Align Source Images" (which greatly increases processing time).
After pressing OK, you will soon see a "Computing Camera Response Curves" message.

Once your computer has stopped processing, it will show a window with their combined
histogram. Photoshop has estimated the white point, but this value often clips the
highlights. You may wish to move the white point slider to the rightmost edge of the
histogram peaks in order to see all highlight detail. This value is for preview purposes only
and will require setting more precisely later. After pressing OK, this leaves you with a 32-bit
HDR image, which can now be saved if required. Note how the image may still appear quite
dark; only once it has been converted into a 16 or 8-bit image (using tonal mapping) will it
begin to look more like the desired result.

At this stage, very few image processing functions can be applied to a 32-bit HDR file, so it is
of little use other than for archival purposes. One function which is available is exposure
adjustment (Image>Adjustments>Exposure). You may wish to try increasing the exposure
to see any hidden shadow detail, or decreasing the exposure to see any hidden highlight
detail.

USING HDR TONAL MAPPING IN PHOTOSHOP CS2

Here we use Adobe Photoshop CS2 to convert the 32-bit HDR image into a 16 or 8-bit LDR
file using tonal mapping. This requires interpretive decisions about the type of tonal
mapping, depending on the subject matter and brightness distribution within the photograph.

Convert into a regular 16-bit image (Image>Mode>16 Bits/Channel…) and you will see the
HDR Conversion tool. The tonal mapping method can be chosen from one of four options,
described below.

This method lets you manually adjust the exposure and

Exposure and
gamma, which serve as the equivalent to brightness and
Gamma
contrast adjustment, respectively.

This method has no options and applies a custom tonal curve,

Highlight
which greatly reduces highlight contrast in order to brighten
Compression
and restore contrast in the rest of the image.

This method attempts to redistribute the HDR histogram into

the contrast range of a normal 16 or 8-bit image. This uses a
Equalize custom tonal curve which spreads out histogram peaks so that
Histogram the histogram becomes more homogenous. It generally works
best for image histograms which have several relatively
narrow peaks with no pixels in between.

This is the most flexible method and probably the one which is
of most use to photographers. Unlike the other three
methods, this one changes how much it brightens or darkens
Local regions on a per-pixel basis (similar to local contrast
Adaptation enhancement). This has the effect of tricking the eye into
thinking that the image has more contrast, which is often
critical in contrast-deprived HDR images. This method also
allows changing the tonal curve to better suit the image.

Before using any of the above methods, one may first wish to set the black and white points
on the image histogram sliders (see "Using Levels in Photoshop" for a background on this
concept). Click on the double arrow next to "Toning Curve and Histogram" to show the
image histogram and sliders.

The remainder of this tutorial focuses on settings related to the "local adaptation" method, as
this is likely the most-used, and provides the greatest degree of flexibility.

CONCEPT: TONAL HIERARCHY & IMAGE CONTRAST

In contrast to the other three conversion methods, the local adaptation method does not
necessarily retain the overall hierarchy of tones. It translates pixel intensities not just with a
single tonal curve, but instead also based on the surrounding pixel values. This means that
unlike using a tonal curve, tones on the histogram are not just stretched and compressed,
but may instead cross positions. Visually, this would mean that some part of the subject
matter which was initially darker than some other part could later acquire the same
brightness or become lighter than that other part-- if even by a small amount.

Final Composite that

Underexposed Photo Overexposed Photo Violates Large-Scale
Tonal Hierarchy

A clear example where global tonal hierarchy is not violated is the example used in the page
on using a GND to extend dynamic range (although this is not how local adaptation works).
In this example, even though the foreground sea foam and rock reflections are actually
darker than the distant ocean surface, the final image renders the distant ocean as being
darker. The key concept here is that over larger image regions our eyes adjust to
 y p g g g y j
changing brightness (such as looking up at a bright sky), while over smaller
distances our eyes do not. Mimicking this characteristic of vision can be thought of as a
goal of the local adaptive method-- particularly for brightness distributions which are more
complex than the simple vertical blend in the ocean sunset above.

An example of a more complex brightness distribution is shown below for three statue
images. We refer to contrast over larger image distances as global contrast, whereas
contrast changes over smaller image distances are termed local contrast. The local
adaptation method attempts to maintain local contrast, while decreasing global contrast
(similar to that performed with the ocean sunset example).

High Global Contrast Low Global Contrast

Original Image
Low Local Contrast High Local Contrast

The above example illustrates visually how local and global contrast impact an image. Note
how the large-scale (global) patches of light and dark are exaggerated for the case of high
global contrast. Conversely, for the case of low global contrast the front of the statue's face
is virtually the same brightness as it's side.

The original image looks fine since all tonal regions are clearly visible, and shown with
sufficient contrast to give it a three-dimensional appearance. Now imagine that we started
with the middle image, which would be an ideal candidate for HDR conversion. Tonal
mapping using local adaptation would likely produce an image similar to the far right image
(although perhaps not as exaggerated), since it retains local contrast while still decreasing
global contrast (thereby retaining texture in the darkest and lightest regions).

HDR CONVERSION USING LOCAL ADAPTATION

The distance which distinguishes between local and global contrast is set using the radius
value. Radius and threshold are similar to the settings for an unsharp mask used for local
contrast enhancement. A high threshold improves local contrast, but also risks inducing halo
artifacts, whereas too low of a radius can make the image appear washed out. For any given
image, it is recommended to adjust each of these to see their effect, since their ideal
combination varies depending on image content.

In addition to the radius and threshold values, images almost always require adjustments to
the tonal curve. This technique is identical to that described in the Photoshop curves tutorial,
where small and gradual changes in the curve's slope are nearly always ideal. This curve is
shown for our doorway example below, yielding the final result.
 Final Result
Photoshop CS2 Tool
Using Local Adaptation Method

HDR images which have been converted into 8 or 16-bit often require touching up in order to
improve their color accuracy. Subtle use of levels and saturation can drastically improve
problem areas in the image. In general, regions which have increased in contrast (a large
slope in the tonal curve) will exhibit an increase in color saturation, whereas the opposite
occurs for a decrease in contrast. Changes in saturation may sometimes be desirable when
brightening shadows, but in most other instances this should be avoided.

The main problem with the local adaptation method is that it cannot distinguish between
incident and reflected light. As a result, it may unnecessarily darken naturally white textures
and brighten darker ones. Be aware of this when choosing the radius and threshold settings
so that this effect can be minimized.

TIP: USING HDR TO REDUCE SHADOW NOISE

Even if your scene does not require more dynamic range, your final photo may still improve
from a side benefit: decreased shadow noise. Ever noticed how digital images always have
more noise in the shadows than in brighter tones? This is because the image's signal to noise
ratio is higher where the image has collected more of a light signal. You can take advantage
of this by combining a properly exposed image with one which has been overexposed.
Photoshop always uses the most exposed image to represent a given tone—thereby
collecting more light in the shadow detail (but without overexposing).

RECOMMENDATIONS
Keep in mind that HDR images are extremely new-- particularly in the field of digital
photography. Existing tools are therefore likely to improve significantly; there is not
currently, and may never be, an automated single-step process which converts all HDR
images into those which look pleasing on screen, or in a print. Good HDR conversions
therefore require significant work and experimentation in order to achieve realistic
and pleasing final images.

Additionally, incorrectly converted or problematic HDR images may appear washed out after
conversion. While re-investigating the conversion settings is recommended as the first
corrective step, touch-up with local contrast enhancement may also yield a more pleasing
result.

As with all new tools, be careful not to overdo their use. Use care when violating the image’s
original tonal hierarchy; do not expect deep shadows to become nearly as light as a bright
sky. In our doorway example, the sunlit building and sky are the brightest objects, and they
stayed that way in our final image. Overdoing editing during HDR conversion easily can
cause the image to lose its sense of realism. Furthermore, HDR should only be used when
necessary; best results can always be achieved by having good lighting to begin with.

Note: In response to multiple emails, no photo within my gallery uses the HDR technique. Only when
necessary, I prefer to use linear and radial graduated neutral density filters to control drastically varying light.
If used properly, these do not induce halo artifacts while still maintaining local contrast. Further, these have
been a standard by landscape photographers for nearly a century. In some situations, however, I can certainly
see when the photo would be unattainable without HDR.

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