External Monitors with High Resolution

PixelPhotography wrote:

Do you know which cable i need to have for this?

There's no cable that will turn one of those iMacs into a hardware monitor for the other. I believe that rkaufmann was suggesting that you buy a standalone external monitor to use with your iMacs. Like the 27" Apple 5K Studio Display (if you have the $$$$$), or a 4K display (if you don't, or if you are in the market for an Eizo-level monitor, and are willing to give up some resolution in return for high-end color calibration / accuracy features).

P. S.

Apple once had a feature (Target Display Mode) to allow reusing an iMac's display. It went away about ten years ago when the first 27" 5K Retina iMac came out, and it didn't return even after the introduction of Thunderbolt 3. Then Apple added restrictions to keep newer Macs from taking advantage of older iMacs that do support TDM.

There are workaround solutions that you could investigate, such as Luna Display (Astropad) and Duet Display, but even if you run a cable in conjunction with them, you're not going to get a first-class hardware-accelerated video connection.

PixelPhotography wrote:

I attempted to connect my secondary iMac (24-inch, 2023 M3) to my primary iMac (24-inch, 2023 M3) as an extended display using screen mirroring. However, the resolution on the secondary iMac is lower than both the primary iMac's resolution and the secondary iMac's native resolution when it is not being used as an extended screen. What could be the issue?

Neither of those Macs support Target Display Mode – or being used as a hardware monitor for another device.

Both of those Macs are new enough to support AirPlay v2, and being AirPlay Receivers for AirPlay to Mac.

Continuity features and requirements on Apple devices - Apple Support

Set up your Mac to be an AirPlay Receiver - Apple Support

I suspect that you have been using one 24" M3 iMac as an AirPlay display for your other 24" M3 iMac.

AirPlay is not as good as a hardware video connection. There may be artifacts and lags. In addition, AirPlay's support for different resolutions is quite limited. As far as I know, it doesn't support resolution settings which exactly match the resolution of the 4.5K iMac screen, and there also might be no ability to use Retina scaling. AirPlay's limitations would easily explain lower-quality results on the iMac being used as the second screen.

Correction:

I see from your first screen capture that the Mac did offer a Retina mode. Unfortunately, it was a fairly useless one ("960 x 540 (HIDPI)) given the actual size and resolution of the target iMac's display.

I believe the original AirPlay was limited to 1080p – and some articles suggest that while AirPlay 2 is able to stream 4K movies to an Apple TV, AirPlay 2 might be limited to 1080p for screen mirroring. Your screen capture does not show any resolution selection higher than 1920 x 1080 (a.k.a. 1080p), which seems to bear that out.

If you are after high resolution then you need a real external display, hate to be blunt but that is the way to go if high quality is important to you.

iMacs are not external displays and never have never intended to so.

The solution, is to get and use a real external display instead of the second iMac.

see > Connect an external display to your iMac - Apple Support

How are you making that connection.

Network solutions do not provide enough bandwidth to support full resolutions.