Can you use a 2017 iMac as a monitor for a MacBook?

[Topkat]

I was told that it is not possible on the 2015 and 2017 models - is that true?

 

[redheeler]

Yes, the Late 2014, 2015 and 2017 models don't have Target Display Mode.

 

[Topkat]

What was their reason for changing it? Courage? It seems like a bad decision to remove a useful function.

 

[theluggage]

What was their reason for changing it? Courage? It seems like a bad decision to remove a useful function.

Simple answer: 5k.

Prior to 2014, the iMac display panel used a standard DisplayPort 1.2 interface internally, so it was probably fairly straightforward to route an external DP signal to the panel. However, because displayPort 1.2 can't support 5k over a single cable, the 5k iMacs used a custom internal interface which, I guess, would have made external DisplayPort support rather more complicated. Plus - at the time, there was no external connection that could support 5k over a single cable.

I'm not sure that the 5k panel supports any mode other than 5120x2880@60Hz anyway - all the iMac resolutions are "scaled modes" that use the iMac's GPU to re-sample the image to the panel's native resolution. I expect the same is true of the 4096x2304 display in the 4k iMac. So they'd have to add in all the circuitry to upscale external 1440p/1080p/30Hz etc. signals to 4k/5k to make it useful.

Theoretically, they could do better now that the latest TB3 interface supports DisplayPort 1.4 & single-cable 5k (...but then maybe the next re-design will go for higher-refresh rates and stick with the nonstandard interface).