Depending on how old the imac is it just won't work....from about 2012, maybe a tad earlier, they won't accept video in.
It's a 2011 iMac with two mini display ports (tried both).
Is there anything in System preferences I need to look at?
@kowarler started
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Depending on how old the imac is it just won't work....from about 2012, maybe a tad earlier, they won't accept video in.