Why can't I switch to 60Hz at 2560x1440 resolution on my Samsung viewfinity 27" 5K monitor with MacBook Pro?

MacBook Pro (13-inch, 2019, Four Thunderbolt 3 ports) - Technical Specifications - Apple Support

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Simultaneously supports full native resolution on the built-in display at millions of colors and:

• One display with 5120-by-2880 resolution at 60Hz at over a billion colors
• Up to two displays with 4096-by-2304 resolution at 60Hz at millions of colors
• Up to two displays with 3840-by-2160 resolution at 60Hz at over a billion colors

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I don't know if you'll be able to use Retina resolutions above "like 2560x1440" with that monitor. You should be able to select Retina "like 2560x1440" or native 5120x2880 (with a Thunderbolt 3 connection), though.

Theoretically. There is a note on Samsung's description of the monitor that I don't like:

"Only Mac computers with Apple silicon introduced after 2020 are supported"

https://www.samsung.com/nz/monitors/high-resolution/viewfinity-s9-27-inch-5k-resolution-ls27c900paexxy/#specs

That's in the glossy section where they are talking about the monitor having Thunderbolt 4. I would have expected a Thunderbolt 4 monitor to be backwards compatible with a Thunderbolt-3-equipped Mac or PC – so you'll have to ask Samsung what's going on with this.

More data points:

https://www.reddit.com/r/macbookpro/comments/16igkz5/why_does_new_samsung_5k_monitor_only_support_macs/?rdt=51122

The people in that thread seemed to be saying that

• They couldn't get the Samsung monitor to work as expected with an Intel MBP that was the same as our OPs, or with another Intel MBP.
• Someone did get it to work with a 13" M1 MBP.
• A Intel MBP like the one that our OP has did seem to work fine with an Apple 27" 5K Studio Display (even though the person who tried the Studio Display ultimately bought a cheaper 4K monitor).

Someone on another site reported that they had to turn HDR off on the Samsung 5K monitor before they were able to select 5K resolution for it on their 2019 MBP. The same person said that their 2019 MBP saw the Samsung as a 5K display when the MBP's lid was open, and as a 4K display when the MBP was in clamshell mode …

If you are in Retina "UI looks like 2560x1440" mode, the Mac should be drawing on a 5120x2880 (5K) canvas and then trying to display that on the monitor. Since you have a 5120x2880 monitor, your Mac will not need to do any downsampling for that final step (the way mine has to downsample from 5120x2880 to 3840x2160).

You can see the "UI looks like" resolution and the internal canvas resolution in System Information (Option –  > System Information…).

as described, that all should work.

are you using a Dock or any additional adapters?

how long is that shipped-in-the-box cable?

what other peripheral are plugged into that thunderbolt Bus?

 menu > about this Mac > (system Report) > ThunderBolt

should be similar to this USB example:

Are you running the same version of macOS in each case?

are you using the same cable in each case?

Have you tried a different cable as a check against broken/sub-standard cable?

official Length spec is less than 0.8 meters for genuine Apple cables, 0.5 meters for other Brands of cables.