A question that pops up over and over on the internet (and it certainly popped up in my mind) is What's the difference between Epson Premium Semigloss Photo Paper and Epson Premium Luster Photo Paper?
It's a darn good question: They're both RC base, both 10 mils thick, both the same opacity. And at first blush, they appear to have the exact same surface.
There are two differences to be aware of (beyond the easy-to-discover "premium semigloss comes in limited sizes"):
1. Premium Luster is much shinier. Where light reflects off the textured surface, you'll get bright white glare. Light on the Premium Semigloss is much more muted and controlled.
2. Premium Semigloss bears a plain "EPSON" backprinting. Premium Luster bears Epson's professional backprinting, reading "EPSON Professional Paper" and "Do Not Duplicate" in multiple languages. (They're both packaged as professional media, so don't ask me why the distinction.)
