Late 2015 iMac and LG Ultrafine 27": Surprise Compatibility (edit: evidently not)

Ledgem

macrumors 68000
Original poster
Jan 18, 2008
1,844
591
Hawaii, USA
Update from March 2020: using SwitchResX, we've determined that this is actually 4K scaled to 5K. Truth be told, I don't know that it matters; having the Ultrafine running in scaled 5K mode, compared with my iMac's true 5K screen, I really can't see a difference. System performance is still excellent. But it's worth mentioning because I'm sure this would matter to someone out there, and I hate having incorrect information available. I'll leave my original post intact, but please be aware.

Here's one finding that I wanted to share, that I didn't find documented anywhere else on the internet.

The basic story is that I really love the 5K display of my late 2015 27" iMac, but I've grown disillusioned with the "all in one" concept after some insects either hatched or crawled under my screen and died there (of course, within a year after my extended AppleCare expired), leaving me with an annoyance despite the fact that this computer still has at least another two years of life left in it. I plan to transition to a Mac mini in the future, and in preparation, came across a nice deal on a LG Ultrafine 27" 5K monitor. Apple's own support page for the Ultrafine states that only iMacs from 2017 and newer can use the Ultrafine at its native resolution; iMacs from 2015 (and 2014) are said to only be able to drive it at a 4K resolution. Fine by me, as this would just be a secondary display.

I bought the Thunderbolt 3 to Thunderbolt 2 adapter and a Thunderbolt cable, and was shocked to find an image that looked just as sharp and life-like as my iMac's display (albeit with a slightly cooler color cast). Windows sizes and fonts all looked the same, as well. If this was what 4K scaling looked like, then who really cared that it wasn't true 5K? But a bit of digging revealed that it probably looked that good because it really was being driven at 5K:
Screen Shot 2019-07-25 at 8.14.33 AM.png

The System Information pane seemed to support this:

Displays:
iMac:
Display Type: Built-In Retina LCD
Resolution: 5120 x 2880 Retina
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No
LG UltraFine:
Resolution: 5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)
UI Looks like: 2560 x 1440
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Display Serial Number: (removed for public posting)
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No
Connection Type: DisplayPort

Refresh rate isn't listed in any of these and Apple has made it very difficult to see what the refresh rate is these days, but mouse and window dragging movement is entirely smooth; I'd be surprised if it weren't running at 60 Hz.

While it's possible that Apple made a mistake with their documentation, I'm guessing it was more a matter of simplification. I chose the maximum upgrades for the CPU and graphics chip when I ordered my iMac, and then maxed out the RAM on my own. The CPU and RAM shouldn't impact this, but the graphics chip would. My guess would be that the other graphics chip options for the late 2015 iMac would be limited to a 4K resolution. Yet Apple's support document would become needlessly complicated if they wrote about what the exceptions were; easier to say that all late 2015 iMacs can't drive it to avoid confusion. Either that, or they never actually verified that this configuration would work.

It's a pleasant surprise. The only downside is that now I might get used to working with two 27" 5K displays, and then when the time comes to move away from my iMac I'll find myself looking to purchase another Ultrafine...

(note to anyone else looking to try this: the Ultrafine comes with a nice, long Thunderbolt 3 cable, but Apple's Thunderbolt adapter is a Thunderbolt 2 to Thunderbolt 3 adapter that requires you purchase a Thunderbolt 2 (or 1, they're the same) cable. Use the 2 meter cables; the 0.5 meter ones aren't long enough to have the monitor and your iMac side-by-side, unless they are facing away from each other. Unfortunately the Thunderbolt ports on both displays are on the left side, so this applies whether you would plan to place the Ultrafine to the left or right of your iMac.)
 
Last edited:

whosthis

macrumors member
Aug 21, 2008
87
21
Thanks for sharing. Keep the TB3 cable, the good ones are really expensive, and you can probably use it in the future.

Are you using it as the main screen now? You might even use the mirroring option and dim the iMac down (or stow it away under the table) - but I guess 2 screens is just too tempting.
 

Ledgem

macrumors 68000
Original poster
Jan 18, 2008
1,844
591
Hawaii, USA
Thanks for sharing. Keep the TB3 cable, the good ones are really expensive, and you can probably use it in the future.

Are you using it as the main screen now? You might even use the mirroring option and dim the iMac down (or stow it away under the table) - but I guess 2 screens is just too tempting.
I'm continuing to use the iMac display as my primary screen at this time. I got used to the insects a lot faster than I thought I would, and while it still bothers me, it's really only in certain use cases that it comes up. I figure I might as well save wear-and-tear on the Ultrafine, as well. And yes, if you have two screens, the temptation to use the dual-monitor setup is too good - particularly as I do use programs that can make use of dual displays.

That leads to one sub-optimal reveal about the Ultrafine specifically, when it's not being used as a primary display. I don't want to just have a static image burning in on it when I don't need two displays, but the Ultrafine doesn't have any physical hardware buttons to power it off. While there are programs for the Mac that can disable other displays and I had read online that doing so would cause the monitor to power down, even with those programs the Ultrafine continues to be powered on, just showing a black screen when disabled. For now, I'm connecting and disconnecting the display's Thunderbolt input depending on whether I need the second screen or not. Others on the internet have taken to using remote power switches to forcefully power down the display when they don't need it. (The display properly powers down when the Mac's display is off.) I knew about the lack of physical hardware buttons going in, but I guess I hadn't thought about it much, particularly as this is my first external monitor completely lacking in buttons and I believed the reports about it shutting off when disabled.
 

whosthis

macrumors member
Aug 21, 2008
87
21
Yes, "disabling" a monitor completely is really a lacking feature.

For example, I have a 2nd screen hooked up, but want to use it only rarely. It has a power "button" on the front and a real switch on the back. Only the switch will make the "disappear" from the system. Especially when hooked up to another machine at the same time, this is a rather poor experience. I do understand that this is not what > 90% of the people would usually want.

What software would disable the screen? I might give it a try here.
 

mikehalloran

macrumors 68000
Oct 14, 2018
1,656
418
The Sillie Con Valley
It has its own power cord that goes to a wall outlet.
Thanks for that.

The power button issue has an easy solution. A power switch can be installed in line with the cable. A power strip is easy and most of us have them lying about but you can get 1–2 socket units, too, as well as short cables from 6"–18".

Inelegant? ugly? Yes but practical and cheap.

Also, something like one if these can be plugged into the end of a short extension cord.
https://www.amazon.com/Belkin-Conse...th+switch&qid=1564092329&s=electronics&sr=1-4

Anyway, it's an easy problem to solve.
 

Ledgem

macrumors 68000
Original poster
Jan 18, 2008
1,844
591
Hawaii, USA
What software would disable the screen? I might give it a try here.
Most people talked about SwitchResX. It has a free ten-day trial with no features withheld, so you can try it for yourself pretty easily and see if it works. It's $16 (or 16 Euros, depending on your location), which seems like a decent asking price if it works for you.
 
  • Like
Reactions: whosthis

whosthis

macrumors member
Aug 21, 2008
87
21
Most people talked about SwitchResX. It has a free ten-day trial with no features withheld, so you can try it for yourself pretty easily and see if it works. It's $16 (or 16 Euros, depending on your location), which seems like a decent asking price if it works for you.
Cool, I used this in the past and have a license already anyway.
 

MiBook84

macrumors regular
Nov 2, 2008
121
58
Malmö, Sweden
MacOS is lying. It does it also on Mac Pro 2013 with tb2 to tb3 converter. The resolution Mimick 5k but Is actually scaled 4k with different sized ui to look 5kish.
 

Ledgem

macrumors 68000
Original poster
Jan 18, 2008
1,844
591
Hawaii, USA
MacOS is lying. It does it also on Mac Pro 2013 with tb2 to tb3 converter. The resolution Mimick 5k but Is actually scaled 4k with different sized ui to look 5kish.
It's pretty convincing when looking at it side by side with my 5K iMac's display - enough that I have a hard time believing that the operating system is lying when it reports the resolution. Do you know of a different way to verify that it's not truly 5K?
 

mikehalloran

macrumors 68000
Oct 14, 2018
1,656
418
The Sillie Con Valley
It's pretty convincing when looking at it side by side with my 5K iMac's display - enough that I have a hard time believing that the operating system is lying when it reports the resolution. Do you know of a different way to verify that it's not truly 5K?
Yea...

I have an iMac Pro and two 27" LG 4K monitors, one on each side. As crisp as the 4Ks are, it's easy to see that the 5K in the middle has better resolution. The 4Ks that I bought were a $2,100 savings over a pair of Ultrafines and I am ok with my choice.
 
Last edited:

Ledgem

macrumors 68000
Original poster
Jan 18, 2008
1,844
591
Hawaii, USA
Yea...

I have an iMac Pro and two 27" LG 4K monitors, one on each side. As crisp as the 4Ks are, it's easy to see that the 5K in the middle has better resolution. The 4Ks that I bought were a $2,100 savings over a pair of Ultrafines and I am ok with my choice.
That's great! But how does that help me prove one way or another that my second 5K monitor is truly running at 5K? MacOS reports it, and it looks identical to my built-in 5K display... all of which seems to indicate to me that it's really running at 5K, despite MiBook84's comment.
 

mikehalloran

macrumors 68000
Oct 14, 2018
1,656
418
The Sillie Con Valley
That's great! But how does that help me prove one way or another that my second 5K monitor is truly running at 5K? MacOS reports it, and it looks identical to my built-in 5K display... all of which seems to indicate to me that it's really running at 5K, despite MiBook84's comment.
If it wasn’t, you’d see the difference.

Believe your eyes and not those who don’t know what they’re talking about.

There are other threads where it has been verified that the UltraFine displays 5K as long as the GPU can handle it and it’s connected via Thunderbolt. The Apple TB3-TB2 adapter works for this.
 

MiBook84

macrumors regular
Nov 2, 2008
121
58
Malmö, Sweden
If it wasn’t, you’d see the difference.

Believe your eyes and not those who don’t know what they’re talking about.

There are other threads where it has been verified that the UltraFine displays 5K as long as the GPU can handle it and it’s connected via Thunderbolt. The Apple TB3-TB2 adapter works for this.
Interesting, I didn’t know that. I have a Mac Pro 2013 (TB2) and connecting my LG UltraFine 5K to it via TB3-TB2 adapter from Apple results in a resolution that is shown as being 5K but in the system profiler its clear that its 4K. They only modify the size of the Ui to make it look like 5K size wise but it’s a tad blurrier.

Afaik the TB2 interface can not on its own handle 5120x2880 pixel bandwidth. Other 5K screens without TB3 utilize dual cables to work around this.

can you please link to those conversations? It would be helpful to me if it was any way to actually get 5K from that computer over one TB cable and adapter.
 
  • Like
Reactions: aclarubicin

Ledgem

macrumors 68000
Original poster
Jan 18, 2008
1,844
591
Hawaii, USA
Interesting, I didn’t know that. I have a Mac Pro 2013 (TB2) and connecting my LG UltraFine 5K to it via TB3-TB2 adapter from Apple results in a resolution that is shown as being 5K but in the system profiler its clear that its 4K. They only modify the size of the Ui to make it look like 5K size wise but it’s a tad blurrier.

Afaik the TB2 interface can not on its own handle 5120x2880 pixel bandwidth. Other 5K screens without TB3 utilize dual cables to work around this.

can you please link to those conversations? It would be helpful to me if it was any way to actually get 5K from that computer over one TB cable and adapter.
What does your system profiler show? I copied and pasted mine (minus the screen serial number) in my first post; does yours look different from that?

For what it's worth, the 2015 iMac also only has Thunderbolt 2. I'm connected through the same TB2 > TB3 adapter from Apple.
 

chorner

macrumors member
Aug 4, 2012
87
37
Toronto, Canada
Any definitive answer on this? I'm very curious if this gives full 5K as I'm close to pulling the trigger on a new iMac 2019 and Apple Pro display - to have a 2nd 5K monitor setup as this old Apple Thunderbolt Display of mine just has to go at this point :)
 

Ledgem

macrumors 68000
Original poster
Jan 18, 2008
1,844
591
Hawaii, USA
Any definitive answer on this? I'm very curious if this gives full 5K as I'm close to pulling the trigger on a new iMac 2019 and Apple Pro display - to have a 2nd 5K monitor setup as this old Apple Thunderbolt Display of mine just has to go at this point :)
Thanks for the message - I actually need to update my post because I had a discussion with some others and, using a third-party program, it's actually not true 5K.

But as for your question, according to Apple any iMac from 2017 and later should support true 5K with these monitors. So, with a 2019 system you wouldn't have an issue.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.