“If you’re looking out a portal, and it’s 700 nits, that’s a load for your eye to respond to. We found ourselves opening up the shadows somewhat to compensate for people’s sensory reaction, where the pupil closes down due to bright light.”
– Shelly Johnson, ASC, explaining how he subdued the contrast in Greyhound (2020)
On his website, Steve Shaw, CEO, Light Illusion, writes that one of the biggest potential pitfalls of PQ-based HDR is that it can be difficult to watch due to excessive eye fatigue. He proceeds to explain that, while the human eye is capable of an enormous dynamic range (the overall range the HVS can adapt to) of as many as 24 stops, we are only able to see a fraction of that figure at any given time. This is known as static dynamic range, and is estimated to be around 12 stops or so. Although the process can take anywhere from a few seconds to several minutes, under normal circumstances, our eyes are able to adjust to changes in brightness without our even being aware of it and we experience no eye strain.
However, Shaw argues, given “the relatively small size of TVs, combined with the standard viewing distance – 3m or so – the whole TV screen is within the high-acuity, central angle of view of the human eye (5° to 15°), meaning the human visual system cannot respond independently to different areas of brightness – being stuck within a state of full adaptation, so the viewer is only able to use the static dynamic range of the human eye.” [In fact, pupil size adaptation can occur when the bright region is as small as 1 degree or even less]. Shaw goes on to say that in order to derive any benefit from HDR, the FOV must be in the neighborhood of 45°. [Sitting at the UHD design viewing distance of 1.6H results in a viewing angle of 65°].
What precisely is this benefit Shaw is referring to, anyhow? While prolonged exposure to excessively bright HDR content can cause eye strain, moderate use of HDR highlights often allows the pupil response to be perceived as a fascinating, non-intrusive aspect of the immersive experience. In real life, our eyes adjust to different lighting conditions. HDR can mimic this by having a higher dynamic range. Pupil contraction mimics the natural response to real-world bright environments, reinforcing the perception of realism. This physiological reaction can heighten the sense of immersion, making viewers feel more “present” in the scene. If lighting ratios are too conservative (i.e., as in SDR), the HDR content won’t trigger the same adaptive response in viewers, making the experience less immersive. Failure to leverage the full potential of HDR inevitably lessens emotional impact.
So, when Shelly Johnson subdues contrast because he objects to pupil contraction, what he is really doing is eradicating one of the principal sensations that makes watching HDR content such an exhilarating experience.
The Daejeon Chronicles 
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