Can I use a 2015 imac as a display for a 2018 MacBook Air with Retina? What is the cheapest way to do this? The thunderbolt adapter is $50, and I'd like to spend less if possible. Thank you!
MacBook Air 13″, macOS 10.12
+1 the 2015 iMac will not work. Please refer to Use your iMac as a display with target display mode - Apple Support and you will see TDM is dead.
If your MBA needs an external display, please buy one. This is a more reliable, less expensive (does not require expensive cabling) and easier solution than TDM ever was. You can easily find external displays that begin at <$100.
Good luck.
No, you cannot use the iMac as a monitor for the MacBook Air. The iMac is not a monitor. The setup you propose is not supported by Apple, neither in hardware nor software.
The best thing to do is buy a monitor for your laptop if you need a larger screen.
2015 iMac as a display for a 2018 MacBook Air with Retina?
