Have you done the Grey Bar Cut & Paste Test on both?
Try It!
Grey Bar Test:
Take a picture with your Camera of the Horizontal Grey Bars & another Picture with the Vertical Grey Bars.
Theses bars can both be found Here:
http://tapplox.com/imac-led3.html
http://tapplox.com/imac-led2.html
Copy Both Pictures from your Camera to your iMac.
Open Up your picture you took in "Preview"
With the Mouse Select the entire Left Grey Bar
Go To File, Copy,
Go To File, Paste
Now Drag the Copied Section to right next to the Right Grey Bar
Go To File, Paste
Go To File, Save As…
Give the Picture a NEW Name
* Do Same with Horizontal Grey Bars.