Old iMac as second monitor

24" and 27" iMacs from that year support Target Display Mode using Mini DisplayPort input. 21.5" iMacs don't. If that was the end of the story, and you had a 24" or 27" iMac, you could use a USB-C to Mini DisplayPort cable.

Unfortunately, Apple put restrictions on the video source. Your MacBook Air is too new to use any iMac as a Target Display. (See: Use your iMac as a display with target display mode - Apple Support).

Your situation can't use the regular Target Display Mode...

The following older iMac models can be used as an external display when plugged into another older Mac model.

iMac models introduced in 2011, 2012, 2013, and mid 2014

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Thunderbolt or Thunderbolt 2 cable.

24-inch and 27-inch iMac models introduced in 2009 and 2010

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Mini DisplayPort cable.

I haven't tried either, but Turn any Mac or iPad into a second display...

https://lunadisplay.com/

Review by NAK005, you may not be happy with it…

Secondary Screen Solution - Apple Community

https://www.duetdisplay.com/

Duet - Remote Desktop, Second Display, Drawing Tablet, and More (duetdisplay.com)

Then Apple has Sidecar & something ele that escapes me right now.

A big +1 on BDAqua's caveat re Luna and others. This advisory is on Luna's main support page:

That wording says to me that they do not intend it to be used to create a primary display. Need a secondary screen to park static windows or palettes? Fine. Otherwise...

Also neither Duet nor Luna are free. The cost of either would go a long way toward the purchase of a decent free-standing external monitor that will be easier to set up, offer full video options, and use far less energy that a running iMac.

I think the other method that BDAqua was trying to recall may have been AirPlay to Mac. A 2009 model is about a decade too old to support that feature, which has its own lag ussues based of videos reviews I've seen showing it in use.

I can see where people would be tempted to use Luna Display to reuse the screens of 27" Retina 5K iMacs and even 24" Retina 4K iMacs – given the relative scarcity, and high cost, of standalone monitors which offer those same combinations of size and resolution. A 27" Apple 5K Studio Display + Apple keyboard and mouse will set you back as much, or more, than some 27" 5K Retina iMacs did. Except that you'll still have to buy a computer!

The case for using Luna Display or Duet Display to salvage 2560x1440 and 1920x1080 iMac screens is not that good, given the cost and drawbacks of the workarounds, versus the cost of buying a new monitor. Why would I want to pay a "subscription" (rental) fee of $48/year to $72/year for Duet Display to reuse a 21.5" iMac screen if $150 will buy me a good 24" 1920x1080 monitor, and $70 will buy me one that's lousy for photos and for movie watching, but that might still do an "acceptable (meh)" job for text?

If I'm going to use that monitor for three years, I'm better off buying a new one even before considering that the old iMac will consume more electricity, and is more likely to suffer a catastrophic failure during that time.

If new Macs could reuse 2011 – Mid 2014 iMacs as Target Displays, the $80 expense for specialized Thunderbolt connection hardware would pose a similar practical barrier to that arrangement.

LessPhonesMoreComputers wrote:

I have a 2015 Imac 5k 27-inch 3.2 gh Quad core and a MacBook Pro M1 Max 32gb from 2021. But what cable would i use to connec them?

Sorry that will not work, because those Mac's do not support the old cabled Target Display Mode.

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

Your best option, is to get and use an aftermarket external display with your MacBook Pro.

see > Use an external display with your MacBook Pro - Apple Support

Re: “I have a 2015 Imac 5k 27-inch 3.2 gh Quad core and a MacBook Pro M1 Max 32gb from 2021. But what cable would i use to connec them?”

That iMac doesn’t support Target Display Mode, and that MBP is too new to support using any iMac as a Target Display.

I have a 2015 Imac 5k 27-inch 3.2 gh Quad core and a MacBook Pro M1 Max 32gb from 2021. But what cable would i use to connec them?

Yes indeed, Airplay, thanks! :)