Which cable?

The one rule

Everything between your Mac and the iMac must carry at least 400 Mbit/s. That is what the sharp, lossless picture needs: every pixel exactly as on your Mac.

Wi-Fi works, but it is compressed. Text gets a little softer while things move and is exact again once the screen is still. A cable that carries 400 Mbit/s or more removes that.

The slowest part counts. One slow adapter, hub or cable in the path, and the whole connection is slow.

Three ways

1. Wi-Fi

   What you need

   Nothing to buy. Both Macs on the same network, on 5 GHz.

   What it costs

   Nothing.

   What you get

   A good picture, compressed while things move and exact again when still. Typically 50 to 400 Mbit/s on 5 GHz, depending on the iMac's radio; far less on 2.4 GHz.

2. Gigabit Ethernet

   What you need

   A USB-C to Gigabit Ethernet adapter for your Mac, one that says Gigabit or 10/100/1000. A Cat5e or Cat6 cable. Straight from the Mac to the iMac, or both through a Gigabit router or switch.

   What it costs

   Roughly €15 to €30 for the adapter and €5 to €15 for the cable. Nothing if your Mac has an Ethernet port.

   What you get

   Up to 1000 Mbit/s: the sharp, lossless picture on every iMac, and Ultra (ProRes) on a 1080p iMac.

3. Thunderbolt

   What you need

   For a 2011 to 2015 iMac with a Thunderbolt port: Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter and a Thunderbolt 2 cable. For a 2017 or later iMac: one Thunderbolt 3 or 4 cable, USB-C on both ends, with the lightning-bolt logo.

   What it costs

   Roughly €55 for Apple's adapter and €30 to €45 for a Thunderbolt 2 cable. A Thunderbolt 3 or 4 cable is €20 to €80, depending on length.

   What you get

   10 Gbit/s or more: the sharp, lossless picture at any size, and Ultra (ProRes) up to 1440p.

The traps

Each of these caps the whole connection below 400 Mbit/s.

• A cheap USB adapter that says 10/100. It does 100 Mbit/s, not enough. It must say Gigabit or 10/100/1000.
• A USB 2.0 hub between the Mac and the adapter. Plug the adapter straight into the Mac.
• An old Cat5 cable from before 2001, without the e. It was made for 100 Mbit/s. Use Cat5e or Cat6.
• Powerline adapters. The box says 1000; the wall gives far less, and it varies by the hour.
• A router with 100 Mbit/s ports. Many older or cheap routers have a 10/100 switch inside. Check the port label, or run the cable straight from the Mac to the iMac.
• One side on Wi-Fi. If the Mac is on Wi-Fi and the iMac on a cable, the Wi-Fi hop sets the speed.

For your iMac

Pick your model and get the shopping list. Every iMac from Late 2009 has Gigabit Ethernet; from 2011 there is a Thunderbolt port too.

How do I know it worked

On the Mac, the iMac's card offers Cable and marks it Recommended as soon as it finds the cable. While the picture goes over the cable, the meter under the card shows the delay, the frame rate and the data rate.

If the adapter is too slow, the card says so: “This adapter only does 100 Mbit/s.” If the cable keeps dropping out, it asks you to plug the adapter straight into the Mac, not into a hub.

Over Wi-Fi, one line under Wireless | Cable tells you when your Mac or the iMac is on 2.4 GHz, when the signal is weak, or when the link is slow.