|xel||minimum exposure||EXIF exposure lower limit||seconds|
|xeh||maximum exposure||EXIF exposure higher limit||seconds|
|xfl||minimum focal length||EXIF focal length lower limit||milimeters|
|xfh||maximum focal length||EXIF focal length higher limit||milimeters|
|xal||minimum aperture||EXIF aperture lower limit||f-number|
|xah||maximum aperture||EXIF aperture higher limit||f-number|
Perhaps there’s a way to specify an exact exposure, focal length, or aperture without having to use both the lower and higher limit keywords together. If so, I haven’t found it yet.
Obviously, not all of the kinks have been worked out. Maybe that’s why these keywords remain undocumented. I have started a discussion thread about this in the flickr ideas group.