For images without exif data, I've given the focal length estimate using the --init_focal_length option. But it remains constant throughout the bundle adjustment process.

Contents of options.txt:
--match_table matches.init.txt
--output bundle.out
--output_dir bundle
--init_focal_length 1078.836
--constrain_focal
--constrain_focal_weight 0.0001
--run_bundle