HDR in Develop Module Basic tab

The short answer is that the HDR button isn’t available because it looks like the ASUS ProArt 279CRV does not meet Adobe standards for HDR photo editing.

Understanding why can get confusing very quickly, because there are multiple standards for viewing HDR but the Adobe requirements for actually editing in HDR are specific. On the ASUS website, the marketing materials for the ASUS ProArt 279CRV mention “HDR” and state that it supports HDR10 and is certified for VESA DisplayHDR™ 400. Sounds good, until you look closer…

The VESA DisplayHDR standards are on a scale from 400 to 1600, so VESA DisplayHDR 400 is the bottom of that scale. The number corresponds to the maximum brightness of the display in cd/㎡ (a.k.a.nits). If you look at the ProArt 279CRV tech specs, it says “Brightness (HDR, Peak) : 400 cd/㎡”, so that confirms that is as bright as the display can be.

And it turns out that is probably the problem. Because, as described in the Adobe article High Dynamic Range Explained, Adobe says that for HDR editing in its photo applications:

Recommended displays include Apple XDR displays, such as a MacBook Pro with an XDR display (2021 or later), and any display VESA-certified as DisplayHDR 1000 or DisplayHDR 1400.

The ASUS ProArt 279CRV can’t achieve DisplayHDR 1000. ASUS does sell other desktop displays that can. The ASUS ProArt Display PA27UCX supports DisplayHDR 1000, and the PA32UCXR supports DisplayHDR 1400. (The Apple XDR displays Adobe mentioned, like the one in my MacBook Pro, can all reach 1000 nits sustained, 1600 nits peak. The HDR button works on that. But the HDR button won’t work on my 27” NEC display because it can’t exceed 300 nits.)

Adobe probably doesn’t enable the HDR button for DisplayHDR 400 because 400 nits is just slightly beyond max SDR white (350 nits), so DisplayHDR 400 barely qualifies as HDR at all. Just enough for them to market it as HDR. When a display can achieve 1000 nits or higher, then you can accurately preview up to four stops above SDR white. Adobe considers that the ideal range for HDR to be worth spending your editing time on.

The HDR button can come on if the display isn’t 1000 nits. I have a 5-year-old iPad Pro with a display that can reach 600 nits. The Lightroom HDR button does work, but the HDR histogram makes it clear that I can’t preview as many stops of HDR highlights as on my 1000-nit MacBook Pro display. So if you can’t afford a 1000+ nit display, something that can do between 500 and 1000 nits can at least let you work with at least a stop or two into HDR highlights.

What about HDR10? That is more of a viewing standard for video delivery, as I understand it. It does support as low as 400 nits, but only for tone-mapping the true edited-in-HDR signal down to fit within that limited 400 nits so it doesn’t look too weird. (While the PA27UCX supports HDR10, the other ASUS models support additional video standards such as Dolby Vision and HLG, but again, I don’t think this category of standards is relevant to editing HDR photos, more for watching video.)