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How to: Use an external monitor with your iMac

25 June 2010 39,544 views 9 Comments

Did you know every Intel-based iMac has a display connector on the back, and that you can quickly and easily attach a second display? Get that old monitor out of the closet — here’s what you need and need to do to start using the display mirroring and spanning functionality built into your Apple all-in-one desktop computer.


If you have a new 21.5 or 27 inch iMac, it comes equipped with a Mini DisplayPort that supports DVI and VGA output. You will need an adaptor and which kind depends on the type of monitor you want to attach (DVI or VGA).

Yes, you can purchase one from Apple (Apple Store, $29). However, a quick check of Amazon reveals the Mini DisplayPort to DVI Adapter Cable ($4.01 + shipping) or Mini DisplayPort to VGA Adaptor ($12.49 + shipping), which get the job for less.

For 2009 and older Intel iMacs, you will need a mini-DVI adaptor. Again, DVI and VGA versions are available from Apple and Amazon’s a good bet for lower cost alternatives.

And, of course, if you’re in a hurry, many Best Buy locations have both Apple-branded and third-party adaptors in stock, but check their site first for local availability.

And, software? Everything you need is already built-in.

Hooking up, turning on

With your iMac turned off, using your new display adaptor, connect the monitor to the display port on the back of the computer. Turn on the display and then the computer.

Your iMac should automatically recognize the new display (System Preferences > Displays). If you don’t see the external display listed, click “Detect Displays.” Alternately, put your Mac to sleep (Apple menu > Sleep) and then wake it.

Once your iMac detects the display, it will automagically choose a resolution. That said, you can choose another resolution, though some of the listed resolutions might not be compatible. If the screen turns black for longer than a minute, try another resolution.

If the display turns to black and won’t come back (a couple minutes), turn it off, put your iMac to sleep, turn on the monitor and wake the Mac to have another go.

More is better

You have another option, as well: the choice between display mirroring and extended desktop (spanning). In a nutshell, mirroring does exactly that — mirrors on the second display what’s on the main display — which can be quite handy for presentations and teaching.

Spanning gives you a bigger desktop that lets you drag Finder and app windows into the second display space, which is really good for monitoring email (see the image above), RSS or parking all of those Photoshop palettes.

Have a suggestion? Sound off in the comments below…


  • norders said:

    Thanks for the article! I’m currently doing this with a Dell 1707FP and it works great except for one thing. When the Mac puts its own display to sleep, it also stops the signal to the external monitor (good), however the Mac seems to keep something alive, as the external monitor never completely shuts off (bad). I think most monitors would just display a blank screen but the Dell goes into some sort of “self test feature” mode, flashing a coloured box around the screen until I physically switch it off or wake the Mac.

    Any ideas?? Thanks!

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  • Robin said:

    I have a Philips monitor connected to my iMac (I’m running 10.4.11). My problem is that I don’t always want the monitor on when the computer is running as it’s only used for certain circumstances. However, when the monitor is off, the iMac insists on placing pages/windows on the switched off monitor when I press cmd N or start up Firefox. This is very frustrating. Do you have a solution for this issue?

    I would be very grateful!

    Thank you in advance


  • FP rocks! said:

    Great advice! Sometimes that is a very needed function. Thanks!

  • Jeff said:

    My Imac screen is full of thin, multi-colored horizontal lines. Quite annoying and dissapointing, but what can you expect from today’s production standards? Any-hoo - hoping to solve the problem with a positive solution - increase my screen size by adding a larger, external monitor. Will those lines appear on the my new screen as well?

  • the rocr (author) said:

    Depends on whether the problem lies with the display or video card. Take it to the Apple Store and they should be able to diagnose it for you on the spot for nothing — your mileage may vary. Yes, charges may apply, but I’ve had very good luck at the Apple Store vis-a-vis folks going the extra mile…

  • Cindy said:

    I have successfully connected a Dell in order to remove all data to recycle the iMac. iMac monitor died everything else works per Apple repair. All the Dell display shows is the iMac start up screen. I’m not savoy and have looked most of the day for a solution. ideas?

  • the rocr (author) said:

    Could be that your iMac is hanging on start up. As soon as you press the power button to boot, press the Shift key and hold it until it either hangs again (drat) or you you see the words “Safe Mood.”

    Good luck.

  • Cindy said:

    Thanks a bunch for your response. Holding shift I don’t have a display on the Dell monitor. Reboot and the OS X display on the Dell returns. Such an ordeal to recycle a computer…

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