Apple made 27‑inch iMacs with a Retina 5K screen from late 2014 to 2020, and the iMac Pro. None of them has Target Display Mode, Apple’s old way of using an iMac as a display. Second Display works on most of them: it gives the Mac you work on an extra display, as if you had plugged in a monitor, and shows it on the iMac.
Which 5K iMacs work
| iMac | Second Display | Connection |
|---|---|---|
| Retina 5K, late 2014 and mid 2015 | No: it can’t run macOS 12.3 | None |
| Retina 5K, late 2015 | Yes | Thunderbolt 2 through Apple’s adapter (not tested yet), or Wi‑Fi and Ethernet |
| Retina 5K, 2017 to 2020, and iMac Pro | Yes | Thunderbolt 3, or Wi‑Fi and Ethernet |
The iMac needs macOS 12.3 Monterey or later, and no account. The Mac you work on, a MacBook or a Mac mini for example, needs macOS 14 Sonoma or later. Which iMacs can be a display
The cable makes the difference
A Thunderbolt cable has room for the whole picture. Each frame of a 24‑inch iMac’s screen is about 17 MB, and 60 of them a second need about 8 Gbit/s: more than Wi‑Fi or an ordinary Ethernet network carries. A 5K screen has more pixels still, so over a network the picture has to be compressed.
- Over Thunderbolt you can choose Highest: lossless, 60 fps, full resolution. Use a port with the ⚡︎ symbol on both Macs. Thunderbolt: which cable and which port?
- Over Wi‑Fi, Ethernet or a USB‑C cable without Thunderbolt the picture is compressed to suit the connection, at Low, Medium or High. Quality levels and the speed test
What we have measured, and what not yet
In our tests a MacBook Pro with M3 Pro sent Highest to an iMac 24‑inch with M1 at 60 frames per second over a Thunderbolt 4 cable, with a median delay of about 9 ms. That iMac isn’t a 5K model: we haven’t measured yet how smoothly older and Intel iMacs show the lossless picture, nor tried the Thunderbolt 2 ports of the late 2015 models. How we measured the delay
You don’t have to guess. During setup a speed test of about five seconds recommends the quality your two Macs can keep up, and the statistics line in the menu shows the frame rate and the delay you really get. The statistics line
A 5K screen over Wi‑Fi
Over a network the picture is compressed with HEVC. Macs without a hardware HEVC decoder, roughly Intel Macs from before 2017, get H.264, which is limited to 4K: on such a Mac a 5K desktop is scaled once to fit. The statistics line in the menu names the stream, HEVC or H.264, so you can see which one your iMac gets. Older Macs and Intel iMacs
The picture isn’t encrypted, so use Wi‑Fi and Ethernet on a network you trust, such as your home. How your screen travels
Sharp text on the 5K screen
- Leave the iMac at its own default resolution. If it runs a scaled one, it says: “For the sharpest picture, use this Mac’s default resolution.”
- On the Mac you work on, choose Larger Text, Default or More Space in the display’s Resolution menu. Default matches the iMac’s own resolution; Retina sharpness and the 60 Hz refresh rate follow the iMac by themselves. Resolution and text size
- The display uses an sRGB-like colour profile. Wide colour (P3) and HDR aren’t supported in this version. Colour
What stays on the Mac you work on
In this version the iMac’s keyboard, mouse and speakers aren’t used. Your Mac’s pointer moves onto the iMac as onto any display, its keyboard types into the windows there, and sound keeps playing on your Mac. Keyboard, mouse and sound
To get the iMac’s own desktop back, turn the display off in the menu of the Mac you work on, or quit Second Display on the iMac. While it waits, the iMac’s screen sleeps like a monitor’s and wakes when your Mac connects. Get back to the iMac’s own desktop
More guides: use an old iMac as a monitor, an iMac as a monitor for a Mac mini and an alternative to Target Display Mode.