Apple advertises on
its website that the iPhone 8 and 8 Plus support "Dolby Vision and HDR10 content."
From that statement, you'd think that these new iPhones would
display HDR in all of its eye-popping glory. But you'd be wrong.
To get the full benefit of HDR content, your device needs to have an HDR display. HDR content requires special display tech to properly render it.
For instance, you'd need a screen with a certain level of brightness (measured in nits), with a certain contrast ratio spec, and wide color gamut display support, to see all the extra improvements.
[...]
This is why you need to buy a brand new TV
with HDR support in order to view HDR content. Your rinky-dink TV just won't do if you want all that extra picture quality.
So what's the big deal? Well, the iPhone 8 and 8 Plus don't have HDR displays. The iPhone X is the only iPhone and Apple device with an HDR screen. Apple says it right there on its iPhone comparison page.
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Normally, if your screen doesn't support HDR, it simply won't be able to read the signal and you'll see the standard non-HDR version.
But that's not the case on the iPhone 8 and 8 Plus. Apple told
Mashable that users of these two phones will see visual enhancements to dynamic range, contrast, and wide color gamut when playing Dolby Vision or HDR 10 content from their respective content providers, but it will not be at the full level of HDR visual fidelity as it'll be on the iPhone X, which does have an HDR screen