Connecting 21” iMac to 3840x2160 monitor

As for Mini DisplayPort to HDMI, there are a ton of adapters. I'd avoid any whose description says "1080p" (that won't be enough for a good 27" or 30" monitor) and look for ones whose description says "4K". They won't give your Mac the ability to drive 4K monitors, but with any luck, they won't get in the way of driving 2.5K ones.

https://www.amazon.com/s?k=mini+displayport+to+hdmi+adapter

If there is no setting in the monitor menu for 2560x1440 resolution, and the Mac and the monitor do not negotiate a setting of 2560x1440 pixels automatically, I think you have your answer.

"No."

I did come across an online manual for a Samsung LS27A800UJUXXU that mentioned a 2560x1440 signal mode – but nothing that I could see about how to force its selection manually.

VGA and DVI are obsolete, or nearly so.

• VGA doesn't support 2560x1440 resolution. You'd be searching for an old, lower-resolution analog monitor.
• DVI requires a dual-link connection for any resolution higher than 1920x1200. The old mDP to dual-link DVI adapters ran from $50 to $200, and Apple's $100 adapter may have been somewhat flaky. If you had a 30" Cinema Display, you'd have to put up with this. For a modern display? Not so much.

Mini DisplayPort to DisplayPort would absolutely work, assuming that the monitor is compatible. The difference between the two is the size of the connector. Adapters are cheap, easy to find, and reliable.

I'm guessing you'd want to look for a monitor

• That is a 27" 2560x11440 or 30" 2560x1600 monitor, with at least one DisplayPort or Mini DisplayPort input.
• That has an IPS panel and near-100% coverage of sRGB. (If no coverage is specified, assume the worst.)

The 30" ones are extinct, or nearly so. If you do find a modern one, you'll pay dearly for the extra bit of vertical space.

If the monitor also has HDMI inputs, USB-C (DisplayPort + Power Delivery + USB) that would let you use it as a docking station for a future laptop, good wide gamut (DCI-P3 or Adobe RGB) coverage, etc., that would be OK. Most modern monitors have at least DP/mDP + HDMI, though there are some that are HDMI-only.

Wg27 wrote:

Thanks again mate.

Would this monitor be compatible do you think? Resolution matches but under Mini DisplayPort it says No, but I wondered if this meant it had no Mini DisplayPort input, rather than being compatible with miniDP to DP?

https://www.pbtech.co.nz/product/MONSAM227600/Samsung-LS27A600U-27-QHD-Business-Monitor-2560x144

Going by the description at that site, that monitor appears to have one full-size DisplayPort v1.2 input, and one HDMI v1.4 input. Under "Accessory" they do not show a DisplayPort cable, which means that you get to buy your own cable.

No Mini DisplayPort inputs, but it's easy to find Mini DisplayPort to DisplayPort cables.

https://www.amazon.com/s?k=mini+displayport+to+displayport+cable

I think the "Mini-Display Gender: No" means that they don't include a Mini DisplayPort to DisplayPort plug adapter like this one (to let you use an existing mDP-to-mDP cable when hooking up the monitor).

They call it a "Business Monitor" and list sRGB coverage as "N/A" (which is kind of funny given how they brag about "stunning colour accuracy" and "incredible depth" – maybe check Samsung's Web site to see if there's more complete information about the monitor there?).

The refresh rate is "up to 75 Hz"; I hope it will adjust to 60 Hz if that is what your Mac offers.

I don't know if there are any third-party utilities you could use on the Mac side to force a 2560x1440 or 1920x1080 selection.

Opps, too late. Ignore the following:

Look in the System/Displays settings pane to see if there's a Scaled or Custom option.