Most home scanners are limited in the size negatives or positive film they accept. They are designed to work with 35mm slides; the size of the lighting strip.
This is a way I found to convert larger negatives and bypassing a scanner. It involves taking a (decently high resolution) photo of the negative itself (on a light board) and then importing the image into Photoshop, and inverting the negative into a positive image.
The video showing my method is on YouTube.
[crarko adds: This is one of those straightforward ideas that people may not think of right away, so I'll go ahead and publish it.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110707164740957