Many will find that when selecting a number of images greater than 99, control-clicking and choosing Slideshow will result in a slideshow that only contains 99 photos. The remaining photos will not be present in the index sheet, nor will they be shown at any time. So instead of using the Finder for large slideshows, use Preview instead:
- Select a range of photos by using a shift-multi select or Command-A.
- Instead of using control-click » Slideshow, double-click on your selection to open the files in Preview. (Drag and drop them to Preview's icon or use the contextual Open With menu if they're not normally opened by Preview.) All of your files should be shown in the drawer (this may take awhile to load for a few hundred photos. The load time can be can be optimized by reducing the size of the preview drawer thumbnails.)
- Select View » Slideshow, and enjoy your massive slideshow!
Important note: For any number of photos greater than 499, the Index Sheet will no longer be available in the full-screen slideshow, although in my testing, the photos above 499 are still seen in the slideshow itself. It appears the authors decided the thumbnails would be too small after 20 rows and 25 columns of entries in the index sheet; probably a reasonable choice.