The Brenizer Method is a way of creating photographs with shallow depth of field not possible with a single shot.
View Ryan Brenizer's instructional video and tips about shooting the Brenizer Method here.
This calculator will tell you the focal length and/or aperture you have effectively accomplished when stitching multiple photos together.
The calculation can be used for any camera format and will give results with numbers relevant to that format (not converted to 35mm equivalent).