Apple Aperture 2 - Manual (Page 521)

Chapter 18 Creating Slideshow Presentations 521
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8 If you want the image display time to be adjusted so that your slideshow is the same
duration as the song, choose “Fit to music” from the Timing pop-up menu.
Aperture adjusts the amount of time each image is displayed to fit the duration of the
selected song.
9 If you are satisfied with your music selection, click OK.
The Run Slideshow dialog appears.
10 Click Start to begin playing the slideshow presentation.