Aperture: How To Construct Impossible Smart Albums: "Aperture’s smart album filtering logic offers only the most basic logical choice: all the conditions set in the dialog or any of the conditions set in the dialog. This means that while there are many different criteria provided, it not apparently possible to combine them in complex ways.
However, due to the fact that there are actually two levels of filtering provided by the thumbnail and grid views, impossible filters can be constructed. For instance, I can find images taken on Wednesday OR Thursday OR Friday AND at between 100 and 130mm focal length. I can find images with the keyword Duck OR Swan OR Goose AND the keyword Lake AND rated two stars and above. And if I want, I can make these type of smart filters specific to a single project or to a collection of projects in a blue folder.