You can offset the shelf sag and use 3/4 ply for the shelves if you wrap
them with a verticle grain face. Make a "frame" for the shelf out of 3/4" x
1 1/4" hardwood and rabbet in for the 3/4" plywood shelf. The shelves will
appear more "massive" but will actually be lighter and more ridgid. Even if
you stick to your 1/2" laminated idea any sort of verticle grain facing is
going to add to rigidity.

Knothead