I am using my Mid 2011 iMac Sierra 10.12.6 as a monitor for my M1 2020 Monterey 12.5.1. I bought apple cables and it works. Except on my 27″ it does not use the whole screen. Just the center 50% or so. Was wondering if it could my apple cables or if there is a setting that I could change. Thank you.
I just like the large screen of my 27″mac. I use many spreadsheets and it is more efficient for me on the imac. Thank you.
I don't think it is supposed to work at all. You are using Target Display mode. If you check out https://support.apple.com/en-us/HT204592 it specifies "The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed."
So maybe you are seeing it partially working, but it definitely is not supported. So I don't think you'll get it to work any better.
My advice: Stop using your old iMac as a screen.
The main reason is resolution. That model has a 2560 by 1440 screen. Even a cheap 4K display you get today would be 3840x2160. And it would be lighter and more energy efficient. Probably better brightness, contrast and quality too. 2011 was 11 years ago and things have moved forward. In a way it is a shame to use a newer M1 Mac with advanced capabilities but with a screen that is 11 years behind. And while the energy savings are hard to measure, they could be significant.
But those points aside, if it isn't working and isn't supposed to work, you don't really have a choice but to recycle the old Mac and use a modern screen.
Thank you for your help. I did not think of it that way.