Editing Multi-exposure HDR capture (section)

{{Short description|Technique to capture HDR images and videos}}
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 {{Technical|date=February 2020}}
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[[File:St Kentigerns Church HDR (8226826999).jpg|thumb|upright=1.2|right|Tone mapped high-dynamic-range (HDR) image of St. Kentigern's Church in [[Blackpool]], Lancashire, England]]

In [[photography]] and [[videography]], '''multi-exposure HDR capture''' is a technique that creates [[high dynamic range]] (HDR) images (or extended [[dynamic range]] images) by taking and combining multiple exposures of the same subject matter at different [[Exposure (photography)|exposures]]. Combining multiple images in this way results in an image with a greater dynamic range than what would be possible by taking one single image. The technique can also be used to capture video by taking and combining multiple exposures for each frame of the video. The term "HDR" is used frequently to refer to the process of creating HDR images from multiple exposures. Many smartphones have an automated HDR feature that relies on [[computational imaging]] techniques to capture and combine multiple exposures.

A single image captured by a camera provides a finite range of [[Luminosity function|luminosity]] inherent to the medium, whether it is a digital sensor or film. Outside this range, tonal information is lost and no features are visible; tones that exceed the range are "burned out" and appear pure white in the brighter areas, while tones that fall below the range are "crushed" and appear pure black in the darker areas. The ratio between the maximum and the minimum tonal values that can be captured in a single image is known as the [[dynamic range]]. In photography, dynamic range is measured in [[exposure value]] (EV) differences, also known as ''stops''.

The human eye's response to light is non-linear: halving the light level does not halve the perceived brightness of a space, it makes it look only slightly dimmer. For most illumination levels, the response is [[Weber–Fechner law#Vision|approximately logarithmic]].<ref name=bhatia>
{{cite book
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</ref> Human eyes [[Pupillary light reflex|adapt fairly rapidly to changes in light]] levels. HDR can thus produce images that look more like what a human sees when looking at the subject.

This technique can be applied to produce images that preserve local contrast for a natural rendering, or exaggerate local contrast for artistic effect. HDR is useful for recording many real-world scenes containing a wider range of brightness than can be captured directly, typically both bright, direct sunlight and deep shadows.<ref name="mann1993" /><ref name="mann1995" /><ref>{{cite book |last1=Reinhard |first1=Erik |last2=Ward |first2=Greg |last3=Pattanaik |first3=Sumanta |last4=Debevec |first4=Paul |title=High Dynamic Range Imaging: Acquisition, Display, and Image-based Lighting |date=2005 |quote=Images that store a depiction of the scene in a range of intensities commensurate with the scene are what we call HDR, or 'radiance maps'. On the other hand, we call images suitable for display with current display technology LDR. |publisher=[[Elsevier]] / Morgan Kaufmann |location=Amsterdam |page=7 |isbn=978-0-12-585263-0}}</ref><ref>{{cite book |last1=Banterle |first1=Francesco |last2=Artusi |first2=Alessandro |last3=Debattista |first3=Kurt |last4=Chalmers |first4=Alan |title=Advanced High Dynamic Range Imaging: Theory and Practice |date=2011 |publisher=AK Peters / CRC Press |isbn=978-156881-719-4}}</ref> Due to the limitations of printing and [[display contrast]], the extended dynamic range of HDR images must be compressed to the range that can be displayed. The method of rendering a high dynamic range image to a standard monitor or printing device is called [[tone mapping]]; it reduces the overall contrast of an HDR image to permit display on devices or prints with lower dynamic range.