HDR Photography Photo Workshop Sample Chapter

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Expand your artistic vision with HDR photography

Text of HDR Photography Photo Workshop Sample Chapter

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    Sample Chapter

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    DYNAMIC RANGE AND DIGITA

    LPHOTO

    GRAPHY

    DYNAMIC RANGE

    WRANGLING WITH EXPOSURE

    SINGLE-EXPOSURE LIMITATIONS

    TRADITIONAL SOLUTIONS

    THE HDR ANSWER

    1CHAPTER

    Pete Carr

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    Although digital photography is a compellingand satisfying endeavor, there are significant limi-tations to the technology. You dont often hear ofthem as such, but they affect you every time youpress the Shutter button. The most serious ofthese limitations, as it relates to High DynamicRange (HDR) photography, is dynamic range.

    The simple truth of the matter is that digitalcameras are unable to fully capture what you seewith your eyes much of the time. The world is adeeply complex subject whose own dynamicrange from dark to light far and away exceeds thedigital cameras ability to record it. To make mat-ters worse, you often limit yourself to an averagedor compromised exposure when you rely on a sin-gle photograph and normal processing techniquesto capture and present everything. This chapterprepares you for the wealth of HDR informationand techniques presented later in the book byillustrating the problems inherent in digital pho-tography today. You learn about dynamic range,exposure, metering, and see examples of howphotographs are typically compromised by limiteddynamic range. You briefly see several traditionalsolutions to these problems so you can comparetheir effectiveness to HDR.

    DYNAMIC RANGEWithin the context of photography, dynamicrange is the range of light, from little to much,that can be measured and recorded, normally by asingle exposure. It is not how little light can bemeasured; nor is it how much light can be meas-ured. It is the difference between the two.Dynamic range is often characterized by theterms exposure value (EV) levels, zones, levels, orstops of range.

    4

    Quantifying dynamic range can be a problematicaffair. There is no official standard, per se; nor docamera manufacturers list each models dynamicrange as part of their specifications. The specificdynamic range of your camera is something youwill have to experience, positively and negatively(see 1-1), firsthand.

    If you understand a few technical factors behindthe scenes, as explained in the following sections,it will help you understand why dynamic rangeoften seems so limited.

    Blown-out details, quite often skies,are the result of limited dynamic

    range. Too much light overexposes parts of the sceneand the camera literally cannot measure any morelight. The resulting image has no details in the overex-posed areas.

    note

    Throughout the course of this book,the context of the term dynamic

    range refers to the range of brightness in a scene. It is,in essence, the contrast ratio of the least illuminatedarea to the most. In a larger sense, dynamic range canrefer to any system where you compare two extremes,such as in audio.

    note

    SENSOR WALKTHROUGHTo understand how a cameras sensor works, ithelps to use an analogy. Begin by imagining abucket. This bucket holds electrons and repre-sents an effective pixel-sized sensor in your digitalcamera. The sensor absorbs photons that strike itduring the exposure and generates an electroncharge that is held in the bucket until the expo-sure ends. The strength of the charge reflects howmuch light was measured. This value is passed toan analog-to-digital (A/D) converter with datafrom the rest of the sensor and turned into animage file.

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    1CHAPTER

    5

    As you might imagine, the size of the bucket(called full-well capacity) dictates how muchlight the sensor can measure. The differencebetween a full bucket and an empty bucket is thetheoretical maximum dynamic range of the sen-sor. The larger the bucket, of course, the greaterthe dynamic range of the system.

    There are a number of factors that reduce a sen-sors dynamic range. These range from electronnoise (see 1-2), which interferes with a sensorsability to record extremely low levels of light, topixel size and sensor efficiency. Simply put, nosensor is perfect.

    BIT DEPTHThe bit depth of a camera is analogous to the stopscale. Each stop doubles or halves light, which isexactly how bits work. Adding a bit to a binarynumber doubles it. Conversely, removing theleading bit of a binary number halves it. Each bit,

    HDR PHOTOGRAPHY PHOTO WORKSHOP / Dynamic Range and DigitalPhotography

    ABOUT THIS PHOTOThis photo was taken outsideon a bright, sunny day of a vintage World War II aircraft.The dynamic range of thescene was too much for thecamera to handle. The darkplane looks fine, but the skyand concrete are too light. (ISO 100, f/9, 1/125 second,Sony 18-70mm f/3.5-5.6 at60mm) Robert Correll

    1-1

    Another factor indirectly affectingdynamic range, and beyond the

    scope of this book, is the linear nature of a digital cam-eras response to light. They do not work as our eyes do.The linear quality of the sensors makes digital camerasincredibly valuable in astrophotography. The directresult is the ability to combine many short exposures,which are more easily attained, into longer ones.

    note

    1-2

    ABOUT THIS PHOTOThis high-ISO shot of jellyfish amplifiessensor noise along with valuable light. (ISO 3200, f/4, 1/20 second, Sony18-70mm f/3.5-5.6 at 18mm) Robert Correll

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    therefore, provides a stops worth of dynamicrange. Thus, a 12-bit camera has 12 stops ofpotential dynamic range, which results in 4096levels of light-to-dark discrimination in eachcolor channel.

    6

    Finally, bit depth can also be used to describe thenumber of colors a camera can capture and storein a file. In this sense, a camera with a greater bitdepth has more color-sensing ability. Each colorchannel of a 12-bit camera can contain up to4096 different shades of that color, resulting in atremendous number of total colors that the threechannels can combine to reproduce. However,dynamic range is the story of brightness, notcolor. As mentioned previously, the shades ofcolor are not evenly distributed across the bit-depth system, which results in a skewed intensityrange for each color. Therefore, having 4096 pos-sible shades of each color is not enough to cap-ture the true dynamic range of a scene withoutcompromise. If it were, you would never needworry about a blue sky turning white.

    If youre wondering where 4096comes from, it is 2 to the 12th

    power. The base is 2 because we are dealing with abinary system. Bit depth provides the exponent. If yourcamera takes 14-bit raw photos, 2 to the 14th powerequals 16,384 levels.

    note

    That may seem like a lot, but unfortunately, evena 12-bit camera which can measure 4096 discreteshades of light in each color channel does nothave the dynamic range necessary to capture asimple scene at the zoo without blowing outhighs and losing details in shadow (see 1-3).

    1-3 ABOUT THIS PHOTOThis photo, processed from a12-bit raw image. (ISO 100,f/5.6, 1/160 second, Sony18-70mm f/3.5-5.6 at 35mm) Robert Correll

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    1CHAPTER

    7

    WRANGLING WITHEXPOSURELike wrestling a steer to the ground in a rodeo,exposure can be a difficult beast to control, espe-cially if you dont want to resort to flash photog-raphy, EV adjustments, changing ISO, or othertechniques. This difficulty is a practical effect oflimited dynamic range.

    EXPOSURE EXPLAINEDExposure is how much light reaches the camerassensor during a single photograph. There aremany ways to manage light and its effect on theprocess, but there are two central ways to controlexposure: shutter speed and aperture.

    Shutter speed. Measured in minutes, seconds,or fractions of a second, shutter speed setshow fast the shutter opens and closes. Thelonger it remains open, the more light makesit into the camera. Faster shutter speeds let inless light. The typical dilemma for a photogra-pher is finding the right shutter speed forscenes with high contrast and movement, asillustrated in 1-4. It is clear that the interiorof the tunnel would need a longer exposure toallow the details to be seen. However, slowingthe shutter speed to take a longer exposurewould increase the amount of light the cam-era senses and not only lighten the inside ofthe tunnel, but it would cause the action inthe scene outside the tunnel to be reduced toa blur. Additionally, the longer exposurewould cause the sky to be blown out.

    HDR PHOTOGRAPHY PHOTO WORKSHOP / Dynamic Range and DigitalPhotography

    1-4

    ABOUT THIS PHOTO Standing in a tunnel, the outside is perfectly exposed, but the image lacksdetail inside the tunnel. (ISO 100, f/4, 1/200 second, Sigma 10-20mm f/4-5.6 at 16mm) Pete Carr

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    Aperture. This describes the size of the open-ing in the lens that focuses light past the openshutter and onto the sensor inside the camera.A larger opening lets more light in and asmaller opening permits less light in.Artistically, larger apertures result in a shal-lower depth of field, blurring deeper area ofthe foreground and background. Along withthe focal length of a lens, the aperture is usedto determine f-stop.

    8

    Manipulating shutter speed and aperture are noless important in HDR than in traditional pho-tography, although you will do so with a differentpurpose in mind. In HDR, you often use a hand-ful of photographs to capture a wider dynamicrange of light than is possible in one exposure.

    Depth of field refers to the size ofthe area you perceive as in focus,

    extending in front of and behind the plane the lens isfocused on (most often the subject). Shallow depths offield have a n