Using 2019 iMac Retina display for new Mac Mini

I am recommended to try a product called Luna Display but it is not available through the App Store and so I don't trust it 100%.

I "trust" Luna, but it simple won't work for what you are trying to do. This notice is quite prominently displayed on Luna's support home page:

The red emphasis is Luna'a, not mine.

That says to me that Luna is intended for letting an extra monitor be a convenient parking place for static items like open Finder windows and app palettes. Neither Luna or any other similar workarounds, including Apple's AirPlay to Mac and network screen sharing, can make your Mac an acceptable primary monitor for your Mini. You need a proper, freestanding external monitor to have a lag-free experience.

Target Display Mode will not work with those year model Mac's.

The 2019 iMac can not be used as a display for the M4 Mac mini using a conventional connection.

see > Use your iMac as a display with target display mode - Apple Support

What you need is an after-market external display or HDMI TV to setup and use with the M4 Mac mini.

see > Connect a display to Mac mini - Apple Support

Then you can use a network connection like Screen Sharing or Luna Display to use the iMac as a second display.

see > Share the screen of another Mac - Apple Support

Unfortunately there is no reliable, and convenient way of using an iMac as a primary screen for a Mac mini these days.

It's simply not supported anymore. Solutions like Luna while perfectly fine to use, not dangerous or have any nefarious issues in any way, simply cannot deliver a lag free experience for use as a primary display.

It will be much easier, quicker and in the long run probably even cheaper to just buy a decent display for the Mac Mini. Good quality displays can be had for under $300 bucks from different retailers. Granted they won't have 5K resolution for under $300, but they will be a good quality 4K in most cases.

For example:

click here ➜ Samsung ViewFinity 4K UHD HDR10 Monitor - Amazon.com ($199 on sale currently)

click here ➜ Dell 27" 120Hz 4K Monitor - Amazon.com ($271)

click here ➜ LG UltraFine 27" 4K Display - Amazon.com ($226)

0752chris wrote:

I rather think you are correct after a few hours trying to make this work - not with Luna but with some workarounds confidently asserted by AI tools. It ought to work and the monitor on the 2019 is studio quality.

The problem is, the 2019 iMac is an all-in-one computer and does not support or have an input to be used as an external display.

Apple seem to have determined that I ought to be able to afford a new monitor!!!!

That is something that should have been researched before buying the M4 Mac mini.

The screen sharing works via the mirroring possibility and a Thunderbolt 5 connection wired. It is however very clunky and not at all user friendly.

That is exactly what Allan and Phil have both been saying.

At this point your options include:

1) Buying an external display for the m4 Mac mini.

2) Return the M4 Mac mini for a full refund if it was purchased less than 14 days ago from Apple.

0752chris wrote:

I rather think you are correct after a few hours trying to make this work - not with Luna but with some workarounds confidently asserted by AI tools. It ought to work and the monitor on the 2019 is studio quality. Apple seem to have determined that I ought to be able to afford a new monitor!!!!

Target Display Mode went away when the first 27" 5K Retina iMac came out in Late 2014.

It is very likely that there were technical reasons for this. Handling uncompressed 5K video in real-time, without frame drops, lags, or artifacts, is an extremely demanding task. The Late 2014 iMacs had Thunderbolt 2 ports – and a single Thunderbolt 2 port might not have had the bandwidth required to accept 5K video.

Even if you could overcome that problem – say, by using both Thunderbolt 2 ports, to accept two signals, one for each half of the display – there could have been other obstacles. In 2017, iMacs got an upgrade to Thunderbolt 3, but Target Display Mode did not return. This could have been an arbitrary marketing decision – but it is also likely that the third-party designers of integrated and discrete GPUs were focused on generating video output, not upon providing the necessary hardware to handle "pass-through" 5K video on a computer with a general-purpose CPU.

You're welcome.

We use both LG and Samsung displays with our Mac mini's.

Set up your Mac mini - Apple Support leads you to Connect a display to Mac mini - Apple Support

The folks at those Apple resellers should know better, but obviously they don't have a clue.

Those crap-spewing AI bots, have no hands on or real world experience.

Apple Partnered with LG to make the higher end Studio Display and the previously Apple sold LG UltraFine displays.

So you get the benefit of that partnership on LG displays that are over a thousand dollars cheaper than Apple's.

Otherwise, Dell and Samsung both offer high quality displays at accessible prices. And unless you need very, very specific color timing and perfect color matching, you really don't have to pay for more expensive options.

0752chris wrote:

Thanks for the links. One Apple reseller recommended the LG. I notice in this upgrade exercise that I have NOT been using the 5k possibilities of the 2019 monitor, having set it to default which is not even 4k. When I try it at 5k I cannot read the text properly even with my reading glasses!

That sounds like a mistaken assumption.

Your eyes are sensitive to physical size. That is why, when you run a 27" 5K monitor at the 5K setting, text and objects shrink and become unreadable. The same thing happens with 27" 4K monitors. When applications are sizing things in terms of pixel counts, and you make pixels smaller and closer together, everything shrinks.

It is very likely that the default for your 27" 5K Retina iMac was Retina "like 2560x1440" mode. Applications will size things "as if" the display has only 2560x1440 pixels. But Retina-aware applications (almost all, these days) will draw letter shapes and fill in photo areas on a (2x2560)x(2x1440), or 5120x2880 pixel, canvas. The result is that you are using the full 5K resolution of the screen – for more sharpness and detail, rather than for cramming more and more, smaller and smaller, stuff onto the screen.

With a 27" 4K monitor running in Retina 'like 2560x1440" mode, the canvas will also have 5120x2880 pixels, but the Mac will have to downscale it to 3840x2160 pixels to display it on the actual screen. Again, the idea is to use extra pixels to increase detail, rather than to cram more stuff onto the screen. But where a 27" 5K monitor has 4 times as many pixels per square inch as. 27" 2.5K one, a 27" 4K monitor only has 2.25x as many.

Re: “Would you recommend staying with the default or setting the max and figuring how to work with it.”

I would not recommend setting the Displays Settings “resolution” to 5120x2880 on that iMac. But you may have other options if you want more workspace.

I’n using a 27” 4K display with a M1 Max Mac Studio. The Mac offers me several Retina modes that use all of the pixels of the display, including “like 1920x1080”, “like 2560x1440”, “like 3008x1692”, and another one that makes things too small. I mostly use “like 2560x1440,” but could live with “like 3008x1692” if I felt a need for more workspace. Non-scaled 3840x2160 (like having a 2x2 stack of 13.5” 1080p monitors)? Forget it!

I’m guessing that your 27” 5K iMac offers some scaled Retina modes between “like 2560x1440” and 5120x2880. You could play with them to see which, if any, offered more workspace without text shrinking so far as to become unreadable.