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.

 

[IngoX]

In Display Preferences you should have a setting to optimize resolution for the iMac or the external display. Have you chosen optimize for the iMac display?

 

[pavinder]

In Display Preferences you should have a setting to optimize resolution for the iMac or the external display. Have you chosen optimize for the iMac display?

View attachment 1940590

Whether I choose optimize for iMac or the other monitor, I still get the same result.
There is a similar preferences window for the second monitor, and I can choose which to optimize there too. But the same result - the iMac display remains cropped to 4:3

 

[IngoX]

Ok. Do you know the native resolution for the other display?

Your iMac should be the Late 2009 I guess, is it the 21.5" (1920x1080) or the 27" (2560x1440) display?

Maybee its a situation where the external display can’t handle the iMacs native resolution?

SwitchresX is an app that can give you more options to chose resolutions that are not presented as default resolurions in macOS, sometimes it solves problems. 10 day trial period.

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

 

[DiskII]

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.

 

[DiskII]

In system preference/display try unchecking mirror display. Then you should be able to select different resolutions for the screens. You can even select the layout setting and drag the menu to the external. Unfortunately in mirror mode the screen changes to the lowest resolution screen so to achieve what you are looking for in mirror mode you would need a different external screen that represents closer to the resolution of the imac.