TIFF_RATIONAL is a "pair of unsigned 32-bit integers" (?) - e.g., http://www.fileformat.info/format/tiff/egff.htm

for now we can set signed (negative) longs as the numerator and denominator (b142):

TIFFField f = new TIFFField(
            new TIFFTag("tag", 12345, 1 << type), type, 1, new long[][]{{-1L, 1L}});