Broadcast TECH


THE SHOOTING RANGE H


Advances in image processing techniques, data compression technology and displays are paving the way for greater use of High Dynamic Range systems. Adrian Pennington reports


igh Dynamic Range (HDR) is claimed by some to be as much of a step change in the viewing experience as the move from black-and-white to colour. It starts with the presumption that existing camera technology is limited in its ability to record the full light range of any scene, with detail in shadows and bright lights particularly susceptible to being clipped or bleached. Capturing lumi- nance essentially depends on the degree of exposure and the ratio of aperture to focal length, or f-stop setting. Film is rated at 16 f-stops of dynamic range and top-end digital cine cameras like the Arri Alexa and Red Epic around 14 f-stops. While ratings above eight are consid-


ered to be of high dynamic range, to achieve anything like the 1,000,000:1 contrast ratio of human vision, at least 20 f-stops is deemed necessary. The science is debated but,


capturing a higher range, this is typically lost through tone mapping – an image processing technique that maps sets of colours together to recreate HDR content for display on screens with a lower dynamic range. “The captured dynamic range off the


camera is reduced to about eight f-stops – although the tone mapping can help preserve some of the perception of the brightness of a captured scene,” he argues. “With HDR, though, you can capture and display more scene information.” Why does this matter? According to Simon


according to Alan Chalmers, professor of visualisation at WMG (Warwick University’s Manufacturing Group) and innova- tion director at GoHDR, 20 is the magic number: “Camera technology has traditionally been unable to capture what the eye can see, resulting in under- or over-exposed parts of the scene,” he says. “HDR allows more detail to be captured per pixel, so there are no more over- or under-exposed pixels. This is a huge advantage since what is the point of having a large number of pixels if all of them are ‘blown out’ because the scene was too bright?” Camera manufacturers are heading towards


Number of f-stops needed to achieve the contrast ratio of human vision. Film is rated at 16 f-stops


20


Robinson, The Foundry’s chief scientist, it reduces the amount of unusable footage: “More stops greatly improves your chance of being able to grade con- sistently to meet a desired look without excessive noise, or to recover footage or detail that would otherwise be lost.” Cinematographers like


International Cinematogra- phers Guild president Steven Poster see the potential. “We don’t necessarily need to show


25-30 stops of dynamic range on screen


but we can use it in post to pull in things that could never be shown before,” he says. “If I can get a camera that does 20 stops of


range, imagine what I can do with it,” says Ted Shilowitz, chief marketer at Red, which is developing sensors to achieve just that. “How can I manipulate images in post to achieve effects that have never been seen before?” From a colourist’s point of view, increased


Spot the difference: two


frames from a video sequence shown at NAB by GoHDR


www.broadcastnow.co.uk/techfacils


HDR with improved sensors. Sony’s F65 has 14 f-stops of latitude, Canon’s C500 offers 12 and Blackmagic Design vouches 13 for its recently launched camera. Red claims its new Dragon sensor affords 15+ native HDR. Even the £150 HD Hero2 from GoPro can capture 11 f-stops. The problem with the current crop of cam- eras, says Chalmers, is that although they are


dynamic range is the most important factor to get the desired image. The Mill colourist James Bamford says: “Film has always dealt with dynamic range far better than digital, but to go higher than 15 stops can only be a good thing. The more detail you have in a picture will only serve to make the image richer and more engaging. The simplest example is interviewing someone up against a window.


w May/June 2012 | Broadcast TECH | 37


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