iMacs since 2009 have all supported ‘target display mode’, which basically lets you hook up your macbook pro using thunderbolt to your iMac and use the iMac as an external monitor for the macbook.
Well.. I was in for a not-so-pleasant surprise , when I realized that this wasn’t supported in Apple’s most recent iMac retina 5K.
A little digging , revealed Apple’s reasoning behind this. Thunderbolt 2 just didn’t have enough bandwidth to support 5K. It currently maxes out at 4K. Thunderbolt 3 (expected to come out with Intel’s skylake ) might support this. But who knows.
target display mode – Alternative?
Until then a nifty little compromise ( as I do ) is to just use thunderbolt2 to network the iMac and MacBook. Assign an IP statically to both endpoints, and run Mac’s sharing over this interface ( Thunderbolt 2 runs at 20Gbps ) instead of your typical 1G network interface.