Datacolor User to User Support Group.

The rationale for the 1.8 mac gamma was that some early Macs that our
grandparents worked on supposedly had a gamma that was natively close
to 1.8.

The way I understand it, the ideal gamma settings are 2.2 because they
are very close to what you normally see on  contemporary display
hardware, no matter if it's a PC or a Mac (their "native" gamma). 

If you set a gamma target that is far from what most hardware natively
has then normally the software needs to use videocard's LUTs to
correct the gamma on all 3 channels. And, umm, corrections on 8 bit
LUTs is not a good idea... So you want do do it as little as possible.

Hopefully posts from experts like David Tobie will put an end to that
1.8 Mac Gamma urban myth.