Which Retina ? The Retina Z2 or the Retina BLT Z3 ?
Which Amiga do you have an A2000 / A1200 with Zorroboard or an A3000/A4000 ?
The Retina Z2 is an ZII card that comes with a custom RTG system. It is not supported by CGX or P96. The mainreason to buy a Retina Z2 today is for parts, as you can use the ZIP-Ram on it as Fast-RAM in an A3000 or on an Oktagon 2008 if you get the 4 MB Version of the Retina Z2.
The Retina BLT Z3 is quite a decent card though it lacks a pass trough for the PAL RGB-signal. The card is supported by CGX and P96. Because it is an Z3 only card you can only use it in an A3000/A4000.
