Can't Get 2560 x 1440 resolution on 27" iMac when in Target Display Mode

[Barnum]

I have a late 2009 27" iMac Core 2 Duo running 10.6.8 Snow Leopard that I would like to use as an external monitor for my core i7 15" mid-2012 Macbook Pro (os x El Capitan) with a NVIDIA GeForce GT 650M 512 MB graphics card (not a retina display). The highest resolution for the macbook is 1440 x 900.

When I use my iMac the resolution is 2560 x 1440 but when I hook it up to my macbook with mini displayport cables in Target Display Mode I can only view the display in 1920x1200, 1680x1050 or 1280x720. Also the screen is very pixelated, especially when it’s scaled to 1920 x 1200.

Is there a way to view the 27" in 2560 x 1440 or will I have to buy a smaller external monitor?

 

[DeltaMac]

While the TDM is set up, go to your MBPro System Preferences/Displays pane.
Go to the Arrangements tab.
Is Mirror Displays checked , or unchecked? If mirroring is checked, then you only have the resolutions that the MBPro internal display allows. Try turning mirroring off, then check the settings pane that will be on your iMac screen. You should have other resolutions available, up to that magical 2560 x 1600 that it is capable of displaying.
If you still do not have 2560 x 1600 available, even with mirroring turned off, try a different MDP to MDP cable.