How to mirror full iMac screen to external monitor with different aspect ratio?

[pavinder]

Hi, hoping someone can help. I have an old 2009 iMac running El Capitan (it can't run any later OS), and the screen keeps blacking out for minutes at a time, so I want to mirror the iMac on a spare old monitor I have with 4:3 aspect ratio.

Connecting via Mini Display port to VGA, mirroring works fine except that no matter what resolution I choose in the Display preferences, the iMac display is automatically cropping to the 4:3 aspect ratio that the other screen uses (there are thick black bands on the left and right of the iMac screen).

What I want is for the iMac to display as normal, but the external monitor to have black bands on the top and bottom (thus mirroring the entire iMac screen).
Is this possible via System Preferences, or if not, is there an app which will make it possible?

Thanks for any suggestions.

 

[Lifeisabeach]

Here's an article that explains how to set the primary display in OS X. It's an older article, but it should apply to El Capitan. Basically, you need to make sure your iMac is set as the primary.

Set the Primary Display on a Dual-Screen Mac Setup

If you are running a dual-display setup, you can easily adjust the primary display monitor in Mac OS X. When would you want to do this? For example, if you have a MacBook Pro 13″ hooked up to…

osxdaily.com

That said, the fact that the screen on your iMac keeps flaking out suggests that this problem may not be fixable. When this happens, the iMac may be switching to that external monitor automatically as using it as the new primary display. If this is the case, you likely are out of luck. But give the tips in the article a whirl to make sure everything is set right.

 

[Slydude]

Those directions should work with El Capitan. Sometimes I find it difficult to grab that little white line to drag to the right place, but it does work.

 

[pavinder]

Here's an article that explains how to set the primary display in OS X. It's an older article, but it should apply to El Capitan. Basically, you need to make sure your iMac is set as the primary.

Set the Primary Display on a Dual-Screen Mac Setup

If you are running a dual-display setup, you can easily adjust the primary display monitor in Mac OS X. When would you want to do this? For example, if you have a MacBook Pro 13″ hooked up to…

osxdaily.com

That said, the fact that the screen on your iMac keeps flaking out suggests that this problem may not be fixable. When this happens, the iMac may be switching to that external monitor automatically as using it as the new primary display. If this is the case, you likely are out of luck. But give the tips in the article a whirl to make sure everything is set right.

Thanks for the link.
I can see how it works when using a two-monitor setup, not sure if it'll work when mirroring the screen. Here's hoping!

 

[Slydude]

If I understand what you want to do correctly, getting the effect you wish will require experimenting with the resolution settings on that second monitor. Open the Displays preference pane. Find the resolution settings for the monitor with the different aspect ratio and adjust the resolution until you get the desired effect. How to Use Multiple Monitors on Your Mac

 

[pm-r]

I can see how it works when using a two-monitor setup, not sure if it'll work when mirroring the screen. Here's hoping!

I'm not sure if it would help provide a solution for you, but I am wondering if SwitchResX wouldn't work.
But he provides a free tryout version you could give a try:

SwitchResX - The Most Versatile Tool For Controlling Screen Resolutions On Your Mac

www.madrau.com

- Patrick
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