Earlier this week, I linked to Stuart Maschwitz’s Twitter thread about Extended Dynamic Range in MacOS. Maschwitz has followed that up with a full blog post explanation that I think is worth reading:
So add a third method of displaying EDR content to Apple’s roster: On these non-HDR displays, Apple has remapped “white” to something less than 255-255-255, leaving headroom for HDR vales, should they be called for. The operating system is complicit in this trickery, so the Digital Color Meter eyedropper shows “white” as 255, as do screenshots.
With Catalina, Apple quietly changed what “white” means for millions of Macs, and none of us noticed.
Think of it this way: This EDR display philosophy is so important to Apple that they are willing to spend battery life on it. When you map “white” down to gray, you have to drive the LED backlight brighter for the same perceived screen brightness, using more power. Apple has your laptop doing this all the time, on the off chance that some HDR pixels come along to occupy that headroom. It’s a huge flex, and a strong sign of Apple’s commitment to an HDR future.
Visit for the explanation of EDR; stick around for Maschwitz’s thoughts on HDR as a tool for videographers.
By the way, this only appears to apply to HDR video — not photos with extended range in the highlights. For example, I took a picture on my iPhone of some food on a platter made of white glossy porcelain and cranked the brightness of it in Photos. It is not an HDR photo, but its white values are brighter than the white background of Photos, even though they apparently share the same RGB values. This is similar to the way HDR video appears on my phone but, unlike video, my 2017 iMac did not show any noticeable difference in the white value of the photo compared to Finder’s white background.