Problem using iMac as an external monitor

Hi,

I have an iMac (27-inch, Late 2012) with OS X 10.10.5, a MacBook Air (13-inch,Mid 2013) with the same version and I wanted to use the imac as an external monitor for the macbook air so I connected a mini displayport to mini displayport cable(from belkin, and I'm sure it's working because I tested it on others monitors) to the thunderbolt port of both macs and pressed cmd and F2(or fn and F2) to convert the imac into an external monitor but didn't work😢.

I also tried to do this on the login window of the imac but same results.

I saw this working in several videos, I don't know what can be the problem.

Thank you