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High-dynamic-range imaging (HDRI or HDR) is a technique used in imaging and photography to reproduce a greater dynamic range of luminosity than is possible with standard digital imaging or photographic techniques. The aim is to present the human eye with a similar range of luminance as that which, through the visual system, is familiar in everyday life. The human eye, through adaptation of the iris (and other methods) adjusts constantly to the broad dynamic changes ubiquitous in our environment. The brain continuously interprets this information so that most of us can see in a wide range of light conditions. Most cameras, on the other hand, cannot. HDR images can represent a greater range of luminance levels than can be achieved using more 'traditional' methods, such as many real-world scenes containing very bright, direct sunlight to extreme shade, or very faint nebulae. This is often achieved by capturing and then combining several different narrower range exposures of the same subject matter.〔〔 Non-HDR cameras take photographs with a limited exposure range, resulting in the loss of detail in highlights or shadows. The two primary types of HDR images are computer renderings and images resulting from merging multiple low-dynamic-range (LDR) or standard-dynamic-range (SDR) photographs. HDR images can also be acquired using special image sensors, like an oversampled binary image sensor. Due to the limitations of printing and display contrast, acquiring an HDR image is only half the story; one must also develop methods of displaying the results. The method of rendering an HDR image to a standard monitor or printing device is called tone mapping. This method reduces the overall contrast of an HDR image to facilitate display on devices or printouts with lower dynamic range, and can be applied to produce images with preserved or exaggerated local contrast for artistic effect. == Photography == In photography, dynamic range is measured in exposure value (EV) differences (known as ''stops''). An increase of one EV, or 'one stop', represents a doubling of the amount of light. Conversely, a decrease of one EV represents a halving of the amount of light. To reveal detail in the darkest shadow requires high exposures. Inversely, to prevent 'bleaching out' of detail in very bright areas, one must choose very low exposures. Most cameras cannot provide this range of exposure values within a single exposure, due to their low dynamic range. High-dynamic-range photographs are generally achieved by capturing multiple standard exposure images, often using exposure bracketing, and then later, merging them (usually within a photo manipulation program) into a single HDR image. Digital images are often encoded in a camera's raw image format, because 8 bit JPEG encoding doesn't offer a great enough range of values to allow fine transitions (and regarding HDR, later introduces undesirable effects due to lossy compression). Any camera that allows manual exposure control can make images for HDR work, although one equipped with auto exposure bracketing (AEB) is far better suited. Images from film cameras are less suitable as they often must be digitized first, so that they can later be processed using software HDR methods. In most imaging devices, the degree of exposure to light applied to the active element (be it film or CCD) can be altered in one of two ways; by either increasing/decreasing the size of the aperture or by increasing/decreasing the time of each exposure. Exposure variation in an HDR set is only done by altering the exposure time and NOT the aperture size; this is because altering the aperture size also affects the depth of field and so the resultant multiple images would be quite different, preventing their final combination into a single HDR image. An important limitation for HDR photography is that any movement between successive images will impede or prevent success in combining them afterwards. Also, as one must create several images (often three or five-sometimes more) to obtain the desired luminance range. Such a full 'set' of images takes extra time. HDR photographers have developed calculation methods and techniques to partially overcome these problems, but the use of a sturdy tripod at least is advised. Some cameras have an auto exposure bracketing (AEB) feature with a far greater dynamic range than others, from the 3 EV of the Canon EOS 40D, to the 18 EV of the Canon EOS-1D Mark II.〔(【引用サイトリンク】url=http://hdr-photography.com/aeb.html )〕 As the popularity of this imaging method grows, several camera manufactures are now offering built-in HDR features. For example, the Pentax K-7 DSLR has an HDR mode that captures an HDR image and outputs (only) a tone mapped JPEG file.〔(【引用サイトリンク】url=http://www.adorama.com/alc/blogarticle/11608 )〕 The Canon PowerShot G12, Canon PowerShot S95 and Canon PowerShot S100 offer similar features in a smaller format. Some smartphones provide HDR modes, and most mobile platforms have apps that provide HDR picture taking.〔(HDR apps for Android ) Google Play〕 Camera characteristics such as gamma curves, sensor resolution, noise, photometric calibration and color calibration affect resulting high-dynamic-range images. Color film negatives and slides consist of multiple film layers that respond to light differently. As a consequence, transparent originals (especially positive slides) feature a very high dynamic range. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「High-dynamic-range imaging」の詳細全文を読む スポンサード リンク
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