In the fall of 2009, Apple retired the 20 and 24-inch iMacs in favor of widescreen models with 21.5 and 27 inch displays.
These machines are not the Retina models we enjoy today, and still packed things like an optical drive. They introduced Target Display Mode, which allowed a user to use an iMac as an external display for another Mac.
Late 2009 and Mid 2010 models required a Mini DisplayPort cable for the feature to work. Setting it up was as simple as running a cable from the Mini DisplayPort on a MacBook Pro over to the same port on the iMac, and pressing Command-F2 on the iMac’s keyboard.
In Mid 2011, Apple put Thunderbolt on the iMac, and Target Display Mode suddenly required the new connector. This setup is more familiar to most users, and many even call the feature “Thunderbolt Display Mode” mistakenly.
(All of this is outlined in a single kbase article, as if you doubted my desire to link to an Apple support page in this post.)
In the fall of 2014, Apple introduced the first Retina iMac, a 27-inch model with a 5K display. It did not support Target Display Mode, as Jason Snell pointed out in his review of the machine:
While recent iMacs have been able to double as an external display via something called Target Display Mode, the Retina iMac can’t. This is cutting edge technology, and pumping this volume of pixels through a display cable or series of cables is a serious challenge. With the Retina iMac, Apple has punted: There’s no better way to attach a computer to a display like this than to build them together. In the future, there will undoubtedly be standalone 5K displays—even from Apple!—and ways to connect them to other Macs effectively. For now, though, the Retina iMac is an island, its own display and computer as one.
When the 21.5-inch iMac with Retina 4k display shipped in 2015, it too dropped this technology. Thunderbolt 2 just couldn’t push these displays in any reliable way.
In short, if you have a Retina iMac, you don’t have Target Display mode.
However, that could be changing in the near future. While Thunderbolt 2 was not up to the task, Thunderbolt 3 is. Found on the new MacBook Pros — and assumedly the next iMac — this port could allow users to turn their 2017 iMacs into big, beautiful displays when needed.1
While the technology would allow Apple to do this, the company would still need to elect to re-enable it in macOS.
When this topic comes up, an argument in favor of Target Display Mode I often hear goes something like this: “It can extend the life of an iMac. It the machine has a failure, you can just use it a display instead of getting rid of it!” I don’t buy this line of thinking, however. Target Display Mode may extend the life of an iMac that isn’t fast enough for the latest software, but if the machine has some sort of catastrophic component failure that keeps it from booting, Target Display Mode isn’t an option.
I cannot imagine that this was a popular feature, but my guess is that people who once relied on it would like the option again. My iMac won’t support it, but in the future when its replacement and my notebook both pack Thunderbolt 3, it could prove useful from time to time.
- The one-port MacBook will be left out of this party. While it ships with USB-C, the port does not utilize Thunderbolt 3. The Intel chipset used in this machine does not support the more robust connection, leading to weird (and confusing) fragmentation. ↩