Will an iMac with 5K Retina Display
drive an external 4K display?
Posted November 21st, 2014 by rob-ART morgan, mad scientist
Updated on March 6th, 2015, with news on the Dell 5K display running at 5K on a Mac Pro.
The iMac with Retina 5K Display specifications page states it "simultaneously supports full native resolution on the built-in display and up to 3840 by 2160 pixels on an external display." We decided to see how well it handled a 4K external display. PowerMax gave us access to a high quality IGZO 4K display, the ASUS PQ321Q 31.5".
INSTALLATION
We connected the DisplayPort of the ASUS 4K display to a Thunderbolt port of the iMac 5K using a DisplayPort-to-mini DisplayPort cable. After restarting the iMac 5K, we now had two displays running at 60Hz. The ASUS was set to 3840x2160 native. The iMac 5K's System Preferences > Displays tile lets you scale the built-in display up to 3200x1800. But if you hold the Option key down and click the word "Scaled," it allows you to choose the maximum 5120x2880.
PERFORMANCE HIT?
What will happen to display performance when there is a GPU intensive app active on both screens?
FurMark (OpenGL)
We ran two copies of FurMark simultaneously at 3840x1080, one on each screen. Each averaged 25 FPS. If we ran one copy at a time, the average was 48 FPS.
LuxMark (OpenCL)
Next we ran two copies of LuxMark simultaneously rendering the Sala scene, one on each screen. Each averaged 790 KSamples/sec. If we ran one copy at a time, the average was 1395 KSamples/sec.
INSIGHTS
You can definitely run a 4K Display at 60Hz on the new iMac with 5K Retina Display. Just keep in mind that one GPU is processing activity on both displays. If you have a static image (like Photoshop menus) on one display and GPU intensive activity on the other display, there will be no performance hit. However, if there is GPU intensive activity on both screens, the performance will drop approximately in half.
I recommend a quality 4K display with IGZO panel like the ASUS PQ321Q. The price has dropped from $3499 when that model was announced in July 2013 to the current price of $1499 MSRP. Remember when we paid $3495 for the 23" Cinema Display (1920x1200) in 2002? Eek! And now you can buy a new Thunderbolt Display (2560x1440) for $999 (which also can act as a second display on the iMac 5K).
March 6, 2015 UPDATE on the Dell 5K display (UP2715K). We have it working at 5120x2880 with a 2010 Mac Pro tower sporting a GeForce GTX 980, the latest web driver, and some special boot-args in nvram. READ ALL ABOUT IT!
I do NOT recommend the cheaper 4K TN panel displays like the Samsung U28D590D and ASUS PB287Q for anything but gaming.
WHERE TO BUY 4K IGZO DISPLAYS
The ASUS PQ321Q 31.5 inch 4K UHD display we tested can be purchased from PowerMax or from Amazon for $1499 or less. There are other brands of 4K IGZO displays like the Sharp PN-K321 and NEC PA322UHD-BK but they are still priced at $3000 and up.
Amazon has the Dell UltraSharp 27" Ultra HD 5K Monitor (UP2715K) at $1999 (or less). Dell Home and Office sells it direct for $2499.99 (search for "UP2715K"). It runs at 3840x2160 with one DisplayPort or mini DisplayPort cable. We have confirmed this with various Mac's using Thunderbolt ports, mini DisplayPorts, and DisplayPorts.
You can get a Mac Pro tower to run at 5120x2880 if you use the dual DisplayPort cables, an NVIDIA Maxwell GPU like the GeForce GTX 980, and the latest Yosemite NVIDIA web driver. READ ALL ABOUT IT!
WHERE TO BUY MEMORY and CPU UPGRADES FOR YOUR iMAC
